It's common practice once the football is over. Better to offer the customer a reduced rate, rather than letting them cancel and the possibilty of them not renewing their sports subs once the season starts again.
The_Lhc is right though, theres no guarantess that because you were offered it, everyone will. Sky's CS agents are limited to the amount of deals they can offer each month, so it's luck of the draw if you get through to an agent who's got deals left to offer.