In principle, I agree with you. The problem is practicability. In the case of your maximum bid of £500 and the bidding in your favour reaches £480, the auctioneer has to call for £500 as the next bid step. If he gets a room bid he has to accept it, in which case what does he do about the commission bid? Alternatively, he could jump to an asking price of £520 in order to ensure that your £500 bid would win if nobody bids, but the problem then is what happens if nobody bids £520. The auctioneer then has to decide if he allows the original £480 bid and doesn't know if a room bidder would have bid £500, thus losing the seller £20 and his own fees, or he knocks it down at £500 and the commission bidder gets charged the extra £20, when the auctioneer doesn't know if a room bidder would have gone to £500 at all. Either way he (the auctioneer) looks dishonest and so I can understand why they tend to operate as I described earlier - it avoids most of the hassle. Also, of course, he's got a million other things to be doing at the same time as trying to sort out max bids v room bids.
Basically, you win some, you lose some. In 20 years of auctions, many in the days before live bidding, I've only ever had one that went wrong with the scenario here, so I'm guessing it isn't a major problem.