Aha, this line in the Wiki explains it: "For World of Hyatt status qualification purposes, the
night credits are associated with the TRANSACTION date of the triggering transaction"
So even if one spends $4,995 in Dec, if the final transaction for $5 is in January, then the 2 night credits will apply in the Jan year. However, this
LITERALLY GOES AGAINST THE T&C's listed on Chase Bank's website which clearly state "
World of Hyatt Tier-Qualifying Night credits will be credited to the calendar year in which the spend occurred." (link here).
Anyway, at least now I know how they do it - not how they say they will do it.