I had a lady increase her price from £12 to £14; she said I'd not put my price up in a few years. It really is only a ten minute job.
I think what happens is that in the past they've lost a good window cleaner; they've just stopped turning up; and if they like you, they'll try to win your favours by giving you more than you ask. It keeps you indebted to them, so you'll keep on providing the obviously excellent service you're providing.
And remember, the customer does not only pay for their windows to be cleaned, they're paying for the tradesman they like. We use a certain plumber, not because he's cheap, but because we trust him.