The answer is lag.

Just because YOU see that you have queued a skill, doesn't mean your opponent does.

Specifically, for you, you will see the skill being queued as soon as you click the *on.

But for your opponent, he will have to wait until:

1. the server receive the request

2. the request is queued into the battle sequence, note that this might be at a later time than you expect as the server could be "ahead" of where you see the battle to be (this is why prompt mystery might not be "prompt")

3. The battle must reach the insertion point from #2 on your opponent's battle animation.

End result? Your opponent have anywhere between 0.5 second to -2 second to interrupt you. Yes, negative time-- by the time your opponent sees #3, it is possible, due to the delay mentioned between server and player in #2, that the battle have already passed that stage in time, ergo negative time. They literally have to act pre-emotively or never get to interrupt.

So it's more or less fair, you are literally "seeing things" due to server lag. Granted, the amount of delay from #2 varies from player to player due to a combination of network and hardware, so it is possible that one player have more time to respond due to that.