When we try to explain it, we say that "because we dont bring in a small machine that needs constantly filling and emptying, and because there are 2 of us - we are much more efficient than someone on their own - so we should be done by approx 1pm as opposed to 3pm. That way you can also get on and do other things in the afternoon".
It is hard though when sometimes they say "oh, you've finished already?" Do we lose any clients because of this misconception of 'I paid ££ and they were done by so and so time, so was I ripped off'? Who knows, but I suspect maybe one or 2 definitely. But then I hope they use someone else and then they say to themselves: "I preferred the other company because they did a better job - and they were quicker, I'll call them again next time" I think we actually had one of our clients do exactly this a while back.
It's a numbers game and you just have to try and do your best.