For anyone that is confused about what a supermax contract is, let me explain it briefly.
When you set the sliders to max salary and set the bonus to a max of 75% for the max number of years, the base salary that shows is not actually 75% of the yearly salary because of rounding errors. The max base salary is actually a few dollars higher per season.
You can manually adjust the base salary to be the max allowed by the game engine (hence supermax), but this doesn't actually provide an advantage to signing the player.