30 hours free childcare query



Hi,
My son is due to start Nursery next month and attend 2 days a week, we are slightly confused regarding the 30 hours.
We have been told out of the 30 hours free funding (stretched over 52 weeks) they deduct 7.3 hours a day and ask us to pay for the non-funded hours of 3.7 hours (11 hours is a full day), with the included wraparound costs.
They state the full day needs to be covered, however I can't understand why the full hours aren't government funded covered?
I understand I would have to pay the wraparound fees which isn't a problem.
My question is, are they limited to only accepting 7.3 hours of government funding or can they take 11 hours a day?
My son is going to Nursery 2 days a week, so 22 hours a week is within quota of the 1140 hours a year.
TIA