Pro rata means “in proportion”.
I frequently see it used in job adverts for part-time positions. The salary will be quoted at ‘£x pro rata’.
What that means is £x is the full-time salary, so if you’re doing 50% of the hours then you’d expect to be paid 50% of the salary quoted in the advert. Because you’re being paid ‘in proportion’.
Now, back to Sky. It will come down to the fact they have fixed billing periods and start with a whole bill. But if you’ve actually started somewhere in the previous billing period, they’ll add it to a later bill instead. So you’re paying your £37 bill for the month, and £26 for the proportionate amount of service you used prior to receiving your first full bill.
As Simon said while I was writing this: