I just tried to buy item 270257064631 which had a reserve amount. I just set myibay to snipe on the item 6 second before the end of the auction with a max. bid amount of $1111 dlls.

myibar make me the top bidder with $910, but didn't make me the winner because I didn't reach the reserve amount. Later I found the reserve ammount was set at $1100.

Why didn't myebay continue to raise my bid until it reached the reserve ammount?

myibay bids up to your maximum amount only ($1111.11 in your case), but the reserve price was actually higher than $1111.11 so you did not win.

You can see more information about how bidding works on eBay by following this link:
http://pages.ebay.com/help/buy/proxy-bidding.html
Please check "Notes" on the bottom.