You are slightly right and I was slightly right. Or vise versa. When you raise your maximum and your current bid increases, the increase is not due to the last increment not being a full increment. It's due to the current bid not being an even multiple of the increment. Frankly that makes much less sense to me, but following is cut and paste from eBay's FAQ on the subject.

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I bid against myself to raise my maximum and the high bid increased. Why?

In most cases bidders cannot outbid themselves. There are two exceptions, however.

1.Let's say you are tied with another bidder, but you are the high bidder because you placed the high bid first. If you place another bid to raise your maximum, you will lose your favored "early bird" status. As a result of placing another later bid, eBay will increase your bid to one bid increment more just so that you will remain the high bidder.

2.If your current high bid is between bid increments and you place another bid, your bid will increase to the next round increment. For example, if you are high bidder at $15.56 and you increase your maximum bid, eBay will round the high bid amount up to $16.00. If your high bid is $15.50 rather than $15.56, the amount will not change (unless of course you are tied with another bidder).