The following is a plausible explanation. Disclaimer: I do not believe the number of anointed is 144 000.
The replacement of an "unfaithful, formerly anointed" individual need not be immediate, but rather, is said to take place after the death of the individual. Thus, the number of recognized anointed members of the "Remnant" will fall both because of deaths of faithful Anointed AND the defection of unfaithful Anointed from the Watch Tower Society. Accordingly, it could be (but need not appear as) a one-for-one replacement because the cumulative defection of not-yet-replaced Evil Slaves would take place over several years, and could be followed by a total replacement in a SINGLE year. If the number of recognized replacements exceeds the number of those who have died faithful since the previous Memorial, the total number of "anointed" will appear to increase. (Of course, such an explanation is necessary only if the number 144 000 be literal.)