Funny Money

I lunched with my two friends the other day;
The bill arrived - we'd thirty pounds to pay.
Each of us paid an equal share - in cash.
Ten coins - times three - collected in a flash
By a waiter who, it seemed, was later told
We'd been overcharged a fiver; so he doled
Us back one coin apiece, and kept the two -
As we hadn't tipped him - well, I ask, would you?
So now, when I recall this dodgy deal,
Of how we each paid nine pounds for our meal:
Three nines are twenty seven - that's quite plain.
The waiter has his two - so please explain
How two pounds, plus twenty seven, once departed
Makes one less than the thirty when we started?