In Eruvin 14b, the gemara asks this, and determines that the 2000 is a wet measurement and 3000 is dry, since dry goods can be heaped above the rim.

לא סלקא דעתך דכתיב (מלכים א ז, כו) אלפים בת יכיל בת כמה הויא שלש סאין דכתיב (יחזקאל מה, יד) מעשר הבת מן הכור דהוה להו שיתא אלפי גריוי
והא כתיב (דברי הימים ב ד, ה) מחזיק בתים שלשת אלפים ההוא לגודשא
אמר אביי שמע מינה האי גודשה תלתא הוי ותנן נמי שידה תיבה ומגדל כוורת הקש וכוורת הקנים ובור ספינה אלכסנדרית אע"פ שיש להן שולים והן מחזיקות ארבעים סאה בלח שהן כוריים ביבש טהורין:

 

The Gemara rejects this possibility: This cannot enter your mind, as it is written with regard to the sea: “And it was a handbreadth thick, and its brim was wrought like the brim of a cup, like the petals of a lily; it contained two thousand bat” (i Kings 7:26). How much is the measure of a bat? Three se’a, as the verse states: “Concerning the ordinance of oil, the bat of oil, you shall offer the tenth part of a bat out of the kor, which is a ḥomer of ten bat, for ten bat are a ḥomer” (Ezekiel 45:14). This proves that the bat is a tenth of a kor, or three se’a, as a kor is thirty se’a. Consequently, the sea, which contained two thousand bat, contained six thousand se’a, the volume of exactly one hundred and fifty ritual baths.
The Gemara asks: Isn’t it written elsewhere with regard to Solomon’s Sea: “It received and held three thousand bat” (ii Chronicles 4:5)? The Gemara answers: That is referring to the heaped measure of dry goods that the sea could hold, as dry goods can be heaped above the brim.
Abaye said: Learn from it that the surplus of dry goods in a vessel relative to liquids is one-third of the contents of the vessel. We also learned the same thing in the following mishna: A carriage, a box, and a cupboard, a round straw barrel, and a round barrel made of reeds, and the cistern of an Alexandrian ship, which is a large vessel placed on a boat and filled with potable water, although these vessels have bottoms, i.e., they are receptacles, since they have a capacity of forty se’a of liquid, which is the equivalent of two kor of dry goods, they are ritually pure. Even if they come into contact with a source of ritual impurity, they do not become impure. Beyond a certain size, containers are no longer considered vessels and, consequently, cannot become ritually impure. This mishna states clearly that a vessel that holds forty se’a of liquids can hold two kor, or sixty se’a, of dry goods.

In Eruvin 14b, the gemara asks this, and determines that the 2000 is a wet measurement and 3000 is dry, since dry goods can be heaped above the rim.

לא סלקא דעתך דכתיב (מלכים א ז, כו) אלפים בת יכיל בת כמה הויא שלש סאין דכתיב (יחזקאל מה, יד) מעשר הבת מן הכור דהוה להו שיתא אלפי גריוי
והא כתיב (דברי הימים ב ד, ה) מחזיק בתים שלשת אלפים ההוא לגודשא
אמר אביי שמע מינה האי גודשה תלתא הוי ותנן נמי שידה תיבה ומגדל כוורת הקש וכוורת הקנים ובור ספינה אלכסנדרית אע"פ שיש להן שולים והן מחזיקות ארבעים סאה בלח שהן כוריים ביבש טהורין:

The Gemara rejects this possibility: This cannot enter your mind, as it is written with regard to the sea: “And it was a handbreadth thick, and its brim was wrought like the brim of a cup, like the petals of a lily; it contained two thousand bat” (i Kings 7:26). How much is the measure of a bat? Three se’a, as the verse states: “Concerning the ordinance of oil, the bat of oil, you shall offer the tenth part of a bat out of the kor, which is a ḥomer of ten bat, for ten bat are a ḥomer” (Ezekiel 45:14). This proves that the bat is a tenth of a kor, or three se’a, as a kor is thirty se’a. Consequently, the sea, which contained two thousand bat, contained six thousand se’a, the volume of exactly one hundred and fifty ritual baths.
The Gemara asks: Isn’t it written elsewhere with regard to Solomon’s Sea: “It received and held three thousand bat” (ii Chronicles 4:5)? The Gemara answers: That is referring to the heaped measure of dry goods that the sea could hold, as dry goods can be heaped above the brim.
Abaye said: Learn from it that the surplus of dry goods in a vessel relative to liquids is one-third of the contents of the vessel. We also learned the same thing in the following mishna: A carriage, a box, and a cupboard, a round straw barrel, and a round barrel made of reeds, and the cistern of an Alexandrian ship, which is a large vessel placed on a boat and filled with potable water, although these vessels have bottoms, i.e., they are receptacles, since they have a capacity of forty se’a of liquid, which is the equivalent of two kor of dry goods, they are ritually pure. Even if they come into contact with a source of ritual impurity, they do not become impure. Beyond a certain size, containers are no longer considered vessels and, consequently, cannot become ritually impure. This mishna states clearly that a vessel that holds forty se’a of liquids can hold two kor, or sixty se’a, of dry goods.