You were still "in the area" therefor you got the proxy kill.
People dont understand that when you are the tower, you are technecly still "in" a object, plane,gv,gun .ect
I would say your timeing was just dumb luck, but the kill is a proxy and is always awarded as such.
I also dont think proxy kills have any impact on your score, but i could be wrong on that one.
Well at any rate, enjoy the kill.
But i dont think it something you or HT needs to loose sleep over, i wouldnt.