There may be another bidder with a higher maximum bid and you are bidding against that.
Many bidder bid only once, but bid their maximum, knowing that eBay will sell to the high bidder at one increment higher than the underbidder.
Look at the times of the other bids. Is there a bidder who bid before you? If he has exactly the same bid but earlier, he wins. If his maximum bid (which you can't see, but eBay does) is higher, the system will outbid you.
For example.
Opening bid is $10.
I like the item and bid $100. It opens to me at $10.
You want it and bid $25. It stays with me at $26.
You really want it and bid $75. It stays with me at $76.
You get hot for it and bid $100.
I win at $100 because I bid first.