The tool shows 0.793 Atk interval for a base atk interval of 2.0s with an atk spd of 679. In game shows 0.9s. Not 0.8s. It doesn't appear to show the rounding mentioned above. Maybe the game is capped at 0.9s?

I tried this with marri. I got her atk spd to a total atk spd of 679 (100 + 579). The tool says this should drop her atk interval to 0.8s, but mine is still at 0.9s