It helps to look at an example here. If the "last block to send bids" is 99, then confirmed bids placed on block 99 would show up as block 100 on block explorers. This can give the illusion that a bid was placed after the bidding period has already ended.
On a relevant note, because Handshake is a distributed system, not all information is distributed in a consistent way at all times. As a consequence, sometimes bids placed on on the very final block of the bidding period aren't shown until the reveal period has already begun. Continuing with the previous example, a bid placed on block 99 may not appear until block 100, and it will seem as if the bid was placed after the bidding period has already ended.
To avoid being affected by last-minute bids, please have a look at the Notes on sniping section of the Handshake auction page in our Learning Center.