I got an item that increases my attack rate, but it's not increasing it by the amount it says it does!
Your interval is being reduced by the proper amount, but there is some math that happens behind that scenes that makes it appear to be giving you less than what you're supposed to be getting.
The issue is that there are essentially two different wordings for your attack speed. Weapons themselves (as well as the character pane) list the attack rate in attacks per second. However, items that affect the attack speed list the difference in time between attacks.
For example, a weapon that attacks once every 2.0 seconds would have an attack rate of 0.50 (as it attacks .5 times per second). An item that makes it go faster might say that it reduces the interval between attacks by -0.1. Now the weapon attacks once every 1.9 seconds, for a total attack rate of 0.52.