Bob, if the first bidder bids $100, it does not automatically go to $100, it starts out at $1 or whatever the minimum bid is. If you then bid $5, it automatically jumps to $6 and the other guy remains the highest bidder. In this case, you would not be the highest bidder unless you bid $101 (or more).
Basically, your bid is saying the maximum you are willing to pay for the item, but only gets that high if someone else is willing to pay that much too.