Failed IF condition is still producing a value that is used by a subsequent ELSEIF function

Summary:

I am using IF/ELSEIF within a fairly tight FIX. The logic is as follows:

FIX(&PeriodVar)

IF(@ISMBR("Jan")) perform X calculation in Jan

ELSEIF(@ISMBR("Feb")) perform Y calculation in Feb that relies on an upper-level data intersection in Jan

For some reason, even if the variable is set to Feb and the first IF (for Jan) returns FALSE, the calculation for Feb uses the number that *would have* calculated for Jan (even though it did not actually calculate and store it in the database)

I feel like I am missing something obvious on how to avoid this situation - any thoughts?