GRAVES ATTACK SPEED BUG

I was playing a normal on EUNE as graves at the jg clear seems to be pretty slow. In the mid game i realized that my Attack Speed on graves with 2 glyphs of Attack speed+ boots attack speed+ phantom dancer was 0.78. I selled Phantom (45% AS) and i had only 0.72. After bought 2 phantoms went to 0.82. Surely there is a bug :)