Well is there anyway you could see what bonus the other person gave?
I offered no bonus in the 1st year (1st year Salary was higher than players yearly average demands, but much lower than the salaries i offered in lower years, contract was heavily backloaded). I can't remember the individual yearly amounts anymore, but I know my total contact offer was more than the other person's contract offer assuming they didn't offer a bonus. If they did, then perhaps that was it...
EDIT: I found where you could check contract details, he didn't offer a bonus either.
I'm very confused now. The only advantage he seems to have is that he gave 5 years as opposed to 4, but I offered more money overall and thus a higher yearly salary. He also gave a mutual option though too.