I was told this a couple of months ago and still cant understand it

3 guys arrive at a hotel after a long drive in their fiesta
They ask for a room. The owner of the hotel says they're is only 1 room left and its £30.
The three men agree that this is acceptable as they can then easily split the bill making it £10 each.
Later, the owner realises that he has over charged the three men.
So he calls the porter and gives him £5 to go and give to the men upstairs.
On his way upstairs. The porter thinks to himself, "How will I split £5 three ways?" So he takes £2 and keeps it in his pocket.
He gives £1 back to each of the guys upstairs.
So if you take that returning £1 off the £10 each paid, they paid £9 each.
but...
9x3=27 + the £2 kept by the porter = £29
Where's the last pound?