I don't know your book's rules on boxing, but I'm going to assume the top one is based on who overall wins the fight (including if it goes to decision, instead of a KO/TKO), while the 2nd one includes the possibility of a tie (so I'm guessing it wouldn't include fights that went to decision).
If you bet a fighter to win from the top offer, you get your money back if there is a draw, a draw is a push.
If you do the same on the bottom offer, you will lose if there is a draw. But, with the bottom offer, you could also bet the draw, and win if it comes.
The pricing has to do with the draw being offered in the 2nd offer. If there is a situation where a tie can lose your bet, then that has to be figured into the probabilities offered, and will lessen the probability, and line, on each fighter to win.