The high bid was not at $19.51 when the high bidder placed his bid of 20.00. Because the high bidder's bid of $20.00 was over the next increment at the time it was placed, it was acceptable. When the high bidder placed his bid of $20.00, the second highest bid of $19.51 was entered automatically by proxy.
I believe that is how it works. Someone may be able to correct me if I am wrong.
I hope this makes sense.
Rob