Stacking Sterak's Gage causes AD bug

I tried a game with Irelia after I realized, that the 25% Base Attack Damage wasn't a unique passive on Sterak's Gage, to see how much damage I could reach with her Q (I had 4 Sterak's and 1 Trinity Force and a Guardian Angel, and my Q could deal nearly 1000 damage, but this aside) - I noticed after a while, that my attack damage didn't rise when I bought several Sterak's. It would just display my base AD (+25%), plus my bonus AD. But if I hovered my mouse over my Attack Damage in the HUD, it would display a much larger number, something like:

AD: 170 Your attacks will deal 250 + 50 = 300 damage.

I think I recorded this with ShadowPlay, but the files are 1 gb+, and I don't know how to cut them down in size or length, since Windows Movie Maker is impossible to find nowadays.