WEEKSINYEAR function [1] counts weeks; and that count generally depends on a convention which year a given week belongs to. E.g., there exists a convention that the first week of a year is the week that includes Jan 1st (so the first week of the year could have 6 days in the previous year); or that the first week of a year is the week which has most of its days in this year; and also the question is, what is the first day of a week (Sunday? Monday?), which can make the difference.

WEEKSINYEAR is implemented to follow ISO 8601 conventions on this [2] (namely, the week starts on Monday; and the first week of a year is the one with most days in this year, which is equal to "the week containing Jan 4th", which is equal to "the week with the first Thursday belonging this year").

This should be reflected in the help (maybe as simple as "The function is based on ISO 8601"). Other date-related add-in functions could also need similar clarifications.

[1] https://help.libreoffice.org/7.5/en-US/text/scalc/01/04060111.html?DbPAR=CALC#bm_id3145237
[2] https://opengrok.libreoffice.org/xref/core/scaddins/source/datefunc/datefunc.cxx?r=dac843c4#616