Hi Xevs and welcome to the forums!

most likely, that is due to latency.

when on your main, you click a target and probably 'fire' immediately. it could be possible that the clone's request when it arrives at the server, could be ahead of the main's notification of a new target.

so... when the command to send a fireball to focustarget arrives from the clone, the server may still have the old target set for the main.. screwing up your attack.

recommendation? check your latency and take a breath before firing