I had a good film from a while ago that shows why collisions happen only on one front end, and how you may have seen his plane fly through your plane (hence "you have collided") when on his front end he was no where near your plane and thus did not collide or receive damage.
In the AvA during the Pacific set, I landed a zeke on the deck of a carrier, and a squaddie landed shortly thereafter. While I waited for the reload, I look to my right, and squaddie is sitting there, a few hundred feet off to my right, hovering in mid air.
I asked him what he was doing, and he said re-arming. I asked where he was, he said right on top of me. I was laughing, and started filming.
His plane starts slowly side-slipping my way as I am watching. It eventually makes it to the carrier (remember, on his front end, he is already on the carrier, and had we been enemies, he would have received the "you have collided" message because he is right on top of me). He gets close enough to where our planes almost touch.
Suddenly, I drop through the darn CV deck, and "crash".
Interesting thing, in reality we were both lagging or something. In the film, BOTH OF US are off the carrier, sitting in mid air as happy as can be -- for a good 2 mins if I remember correctly. And at no point did he and I have the same perception of where the other one was.
So again, you get damaged and die because your computer decided you were both in the same place at the same time. His computer may believe something VERY different.