ability queue with casters?

I saw that on my BLM if I use blizzard 3 like right in the middle of the cast bar the new one will ending up casting after it fires off. So is this how the ability queue works or is it part of the delay thing going on?

That is the same as my BLM. Even happens with Fire III.

I saw it happening with other abilities as well such as rune if you press it it before it finishes casting.

I believe it has to do with the delay between the client and server. I recall there's a slight delay between when you press a button, and when the casting bar appears. This same delay is how soon you can press the button before the casting bar disappears and have it "queue up"