Ok thanks for the answers. I really don't think there was a "matching" bug. I had like 150 Army Power (Don't know the name) and the other had like 96. The problem was that the dragon alone did all the job. It was a royal dragon i think. I don't care about losing but when one unit do all the job you kind of question yourself about the balancing of the units in game. The dragon can be strong, ok, but to run faster than horses makes it impossible to catch while he keep throwing fireballs at you.
What I was really intrigued is that I was placed to defend something that was not mine. I don't know if others players can ambush your army but if not then there was a real bug there