WOW! - took the daughter to Alton Towers for 2 days and look at this thread!
Sunshine has it in a nutshell - basically the price for a house is the longer period option ie £15 but the custy would like
4 wky so in fact they get a discount rate for having a more frequent service and that is £10
therefore those with a sliding price scale give better service and value for money than those charging X regardless of length as they must be overcharging all their 4 wky custy's
As Gold pointed out the car wash thing is automatic - sorry every house is given personal attention and i see a big difference in 4 to 8 weekly cleans. and have you seen how cr@p a car wash is on a really dirty car.
It appears that some w/c's are not confident with their business/work as to worry about the 'next' flyer taking their business,
my experience so far has been that the majority of customer moves are due to bad work or being unreliable - otherwise custy's tend to stick with you through thick and thin.
Darran