Why in PvP when I'm fighting someone and they are low they always seem to turn it around at under 5% HP and always kill me when Im above 80% and I don't know how this works...

This literally makes no sense to me. I was fighiting a mage as a druid and he was one hit away, all throughout his HP it seemed to go down fast. But nope they someone get back up enough to one shot me. Is it that I play boomy? I can't understand it.