Sky Mobile Bill - plan and bill don't match?

[faxfan2002]

When I go into my Sky App I see the following -

Mobile Bill -> Plans -> SIM Name -> My Mix 1 Aug - 31 Aug - £8 for 2GB data

Your SIMs -> SIM Name -> Manage SIM -> Data & Plan Settings -> £6 for 2GB data

I have 3 SIM's and all being charged at more than Data & Plan settings (2GB is + £2 extra and 4GB is £1 extra).

Any ideas why this might be? Some overlap on billing periods?