PI Mismatch After Upgrade

Has anyone encountered this: You install an upgrade that says that the performance index will max out at 900. After installation, however, the vehicle eds up with a PI of 901 or 902, bumping it into the next car class. This has happened to me on back to back upgrades on two different vehicles. Any suggestions greatly appreciated!

Aspiration glitch?

Have you tired heavier rims to bring the PI back down?