Some of the extensions are multiplied, not added.
Basically, you dont take the full % of the extensions all at once, you multiply them for each level.
100% x .99
99% x .99
etc etc etc
It makes the bonus less effective in the end, but really doesnt add much "depth" to this area of the game. Where it helps is when you stack damage tunings or something of the like, where each additional tuning adds 5% over 105%, then 5% over 105.25%, etc.
I'm not positive this is whats happening with that extension, I'm just offering up a possible explanation. You might want to check the math to see if it fits.