I found your thread from google and I believe it is the only INTCK vs. YRDIF thread and I am having the same issue while reading the base exam prep guide.
INTCK counts +1 every time it encounters 1 Jan.
YRDIF counts +1 for every 365 (or 360 or 366 days depending on the optional third argument you use, I don't know how the 30/360 work tho)
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