Some WGs put little thought into their rates above an hour, so what they put for 2 or 3 hours isn't necessarily what they would stick to as I know when haggling with regulars. I haggle on bookings of over 2 hours which in my case means 3 hours on occasion 4 hours. The price most certainly isn't always the price in punting dependent on the WG as ever.
I have seen rates that make no sense at all but it is true to say some price them in the hope of attracting punters to book what suits the WG. If I was a 1 popper I would probably do half hour punts but I am not so my minimum is an hour. And its usually the case most WGs half hour rate isn't exactly half her hour rate, this is to encourage punters to book an hour not half hour obviously. So for me a good hours punt is better VFM to me than a good half hour punt where I cant do what I want to do in the time.
So for some WGs there is A method to their pricing which maybe only they know.