"syncing" is just a lazy way of typing synchronizing. In Halo's case, when you're playing online, most things have to synchronize in order for every player to see nearly the same changing surroundings as other players (I say "nearly" because latency makes that near-impossible.)

AI just isn't synchronized online because the net code doesn't include sending or receiving any information about AI-controlled bipeds. That means that the only AI you are going to see correctly are your AI, while every other player's AI could be anywhere else doing anything else, including the host's.

AI maps, although still fun, can be frustrating because of this. The host's AI are the only AI that can truly harm you, and yet there's no way of knowing where they are, whereas the AI that you do see could be shooting you like hell and yet it doesn't affect you at all.

So, if you see a friend shooting at nothing, don't ask. You know why.

Basically it means AI in multiplayer i pointless.