In Hilchot Tefillin 4:26 Rambam writes:
וכל שאינו מניח תפילין עובר בשמונה עשה שהרי בארבע פרשיות צוה על תפילין של ראש ועל תפילין של יד
Whoever does not wear tefillin transgresses eight positive commandments, for in each of the four passages contained in the tefillin we are commanded to wear both head and arm tefillin. (Touger translation)
The commentaries point out that this comes from a Talmudic passage in Menachot 44a:
אמר רב ששת כל שאינו מניח תפילין עובר בשמונה עשה וכל שאין לו ציצית בבגדו עובר בחמשה עשה וכל כהן שאינו עולה לדוכן עובר בג' עשה כל שאין לו מזוזה בפתחו עובר בשני עשה וכתבתם וכתבתם
R. Shesheth said, Whosoever does not put on the tefillin transgresses eight precepts; and whosoever has not zizith attached to his garment transgresses five precepts; and every priest who does not go up to the platform transgresses three precepts; and whosoever has not a mezuzah on his door transgresses two precepts, namely, And thou shalt write them, And thou shalt write them. (Soncino translation)
If we look at Hilchot Birkat Kohanim 15:12 we see that Rambam mentions the number of mitzvot violated there as well:
Any priest who does not ascend to the duchan - even though he neglects [the performance] of [only] one commandment - is considered as if he violated three positive commandments, as [Numbers 6:23-27] states: "This is how you shall bless the children of Israel," "Say to them," "And you shall set My name..." (Touger translation)
However, in Hilchot Tzitzit he does not mention how many mitzvot are violated by not wearing tzitzit, and in Hilchot Mezuzah he does not mention how many mitzvot are violated by not having a mezuzah.
Given that all four cases stem from the same Talmudic passage, why does Rambam only mention this in two out of the four cases?