I'm glad we agree that causing someone physical damage is worse.
For the record, I'm not the one saying it was the drunk person's fault (that's another discussion going on in this thread) and I do think the guy should be punished, but not given prison time. Most of you have no idea what prison is like, especially for someone who isn't your everyday smack head/mugger/GBH repeat offender. Depending on his character, he might need counselling, anti-depressants, might even come out a drug addict given that 80% of prisoners take illegal drugs.
Answer me this: if you expect to go to prison for this, why isn't the prison system full of "t-bag offenders". Because this sort of "prank" occurs all the time. Shouldn't they all be in prison? Once again, my problem is with selective justice.
We don't agree at all but nice try at the cheap debate trick.
As for your question at the end, one would imagine the difference in punishment has arisen from a few subtle differences to many cases, that it happened to someone he didn't know well and thus couldn't make any reasonable assumption of mutual understanding of humour, the fact it got videoed and finally that he was showing the video to people, presumably in some sort of pathetic attempt to look cool to his peers.