Thunderlord's Decree

So this is just something I noticed. The tooltip for Thunderlord's says that it will proc on the 3rd attack or damaging spell you do on an enemy and then goes on cooldown. I notice that whenever I play, it looks like it only procs on the second spell. I'm not sure if there's something I'm missing or if there's a small bug going on. Let's take Lux for an example. I land a Q on the enemy and it binds them. If I autoattack them afterwards, Thunderlord's activates. I'm guessing it might be from her passive, but I'm not really sure. Here's another example: I'll play Annie and let's say I stun someone with my Q or R. If I W or autoattack after that, it will proc Thunderlord's again, but this time, I don't think her passive has anything to do with it because I would have already used it to stun them anyways. Could someone please explain if there's some sort of hidden calculation I'm missing or such? I've had this happen before with other champs like Lulu as well.