I've sometimes noticed this, and have been meaning to mention it. I'm seeing something very similar. I'm not sure, but the game sometimes seems to 'save up' the money, then add it in bulk. For instance, if I collect a duplicate common collectible, no $5 gets added, but if I then pick up an uncommon one, the total of $20 does get added. It's not always as simple as that, as I never really know what the peeps are earning for each payment, but the bank account usually looks OK, more or less.