Evaluation no longer tracks evaluation of main variant

If I remember correctly, until recently the evaluation of the position (the big number at the top left of the analysis display) always coincided with the evaluation of the main alternative (the small number to the left of the top alternative). If there was ever any discrepancy between these two numbers, it used to persist only for a fraction of a second. Now the evaluation of the position sometimes stays at a value different from the evaluation of the main variant for much longer durations, sometimes up to something like ten seconds. (When the analysis terminates, the numbers always coincide.) I'm wondering whether this is a bug or whether there's a reason for these discrepancies.