It looks like the offer he took was the better offer. The max 75% is based on the first year, not the entire contract - it is impossible to get to 75% of the whole contract because the minimum increase of base salary is 5% each year, so years 2+ will be less than the 75%. So if you look at the first year base + bonus, it is $13,230,000, 75% of this is $9,922,500 which is what he signed for.
The difference is the increase of the base salary, the default (and minimum) is 5% per year, but that offer has an increase of 8% per year. If there is a tie with the guaranteed money, the base salary breaks the tie. You can override the base salary values on the contract form, I do need to look and see why the AI did it by that much though.
To answer the question about who wins if all other things are equal, right now it's random. There will eventually be other considerations, but there are none right now. So the best way in a heated max-contract situation would be to offer more base salary in future seasons - but be careful because you don't want to get stuck with a huge future contract that he won't renegotiate out of.
