If you are bidding on an auction that has a reserve price, and your bid is high enough to meet that reserve price, then eBay (not AS) automatically registers your bid as the reserve price.
Say an auction has a reserve price that has not yet been met. The reserve price (which is not known to bidders) is set at $50. The current high bid is $28. If your snipe is for $75 and no one else bids, then you would win the auction for $50, which was the minimum reserve price needed to win. In other words, eBay automatically upped your bid, past the next increment of $29, all the way to the $50 reserve price, because that was necessary to be the winning bidder.
If, on the other hand, your bid was for $40, then eBay would simply register the bid as $40 and you would be listed as high bidder, but the reserve would still be unmet. The auction would either close without a winning bidder, or someone else who bid $50 or more would win.
Remember, it is eBay, and not AS, that registers bids and automatically ups them to the reserve price, where possible. AS only places the bids; it is eBay who registers them.
Some snipers would tell you that, in such an auction, they prefer to bid manually earlier, until the reserve price is met, then stop bidding and let others take the high bids. Then, no longer worrying themselves about meeting the reserve price, they would then set a snipe for the last minute, as per usual. But I disagree. To me, this tips off competitors that you have interest, and they may plan higher bids themselves, particularly late in the auction.
I treat a reserve price like any other snipe: I decide what is the absolute most I would be willing to pay under any circumstances, then set that snipe. If I am outbid, or my snipe is not high enough to meet the reserve price, then I accept that fact and move on. The item will come around again someday.