Practice Mode is not quite optimized for "endless last hitting" like advertized.
So as I finished up my exams and now have the summer to myself to try hard again I decided I'm going to get good. Well that means starting at fundamentals right? so I go into shiny new practice tool, lock XP level, turn off damage to towers and go bot lane to practice. over time I noticed myself missing more and more and thought I was just off. Then I noticed I couldn't last hit under tower anymore with the normal fundamentals. (two hits for melee, hit once for ranged > let tower hit > hit again after). Then I did some research on the LOL wiki. Basically every 90 seconds minions get stronger. Whether or not this means health I'm not sure, but assuming I was able to last hit under tower at the start of the game and could no longer do so later makes me believe so.
It would be a great change to lock MINION strength as well for practice tool so you can just stay bot and keep practicing without having to reset the game every 10 minutes or so. re teleport, spawn minions, etc. This way you could truly endlessly last hit.
Just a suggestion!