RR3 Wiki:Upgrades Section (Classic Version)

There is a long standing problem with PR values, they change (+/-0.1) depending on the order the upgrade is performed in, therefore if there is a 0.1 difference there is no need to change the table.

You can read Admin Kuzz's explanation here: User_blog:Kuzz/The_Pulse_-_Week_4/2015#Performance_ratings and the comments :)

One explanation was posted by :208.184.33.66

208.184.33.66 wrote: I think that the underlying issue with the numbers is that the game actually calculates them to a further decimal place than it shows on the screen (think about how the distance works in endurance races, for example, where during the race you see your distance to two decimal places but the reward rounds it to the next lowest 10th of a mile/km). Anyway, I think what happens is, for example, the game may assign a .44 PR rating to a particular upgrade. If your current PR before that upgrade is added is a whole number (or, perhaps a whole number plus a tenth, depending on how it deals with rounding of .5), the game will show that the impact of the increase will be .4. However, if you have anything higher hat has not already been rounded up in the PR, the game will show at that point that the marginal impact of the upgrade is .5.