Talmud Rosh Hashanah 32a says:
מתני' אין פוחתין מעשרה מלכיות מעשרה זכרונות מעשרה שופרות
We do not lessen (saying) 10 verses in each of Malchuyot, Zichront and Shofarot (The middle 3 sections of Musaph Rosh Hashannah).
The Gemarah, there, discusses the breakdown, and it decides a practice to say 3 citations from each of the 3 sections of Tanach, with a concluding citation from the Torah.
Granted, that while it says that we should not do less than 10 verses in total, we cite exactly 10 verses, in both Malchuyot and Zichronot. However, in the Shofarot section, we cite 4 verses from Ketuvim (Writings) with the last one being all of Psalm 150.
Not only does that make that section 11 verses, but it's more than 3 in Ketuvim.
While, it seems that according to the Gemarrah this is fine, why do we make an exception only for Shofarot section, and why are we specifically adding an extra citation just for Ketuvim? Why not make an extra Torah or Nevi'im citation, instead?