Many early games (before End of Ball bonus collect), do not reset playfield bonus between balls and/or game resets. Look at it as an incentive to drop another coin to prevent someone else from collecting your hard earned points!
I think it was even brought up that on Williams Lucky Ace (Single player version of Dealer's Choice) didn't clear the 10 to Ace sequence between games (1-9 IS reset with each new game). That one definitely struck me as a bit odd.