So, while poking around on the PBE, I found that when you have excessive numbers of Doran's Blades, the value of the life return is not consistent. In other words, it should always return 20 health at 4, 25 at 5 and 30 at 6. I'm not sure, and unable to tell, if this is the case with fewer numbers of Doran's Blades, but here is what happened.
I bought 6 Doran's Blades using both Frosted Ezreal and Resistance Caitlyn, then proceeded to get myself to less than half health via taking turret hits. I proceeded to auto attack various minions and monsters, to see inconsistent values returned with Doran's Blades. Most of the time, it would return 30 or 31 health, but sometimes no value shown (so less than 20) or 22 (if it shows a number less than 30, it always shows 22 at 6 Blades) health returned. This is most noticeable on monsters with higher health values (Ancient Golem, Lizard Elder, Dragon, double golems).
1. Purchase 6 Doran's Blades (also works with 5)
2. Get to less than full health by any means
3. Auto attack minions/monsters, notice inconsistent health return values.