Julie is right, until last year I working in the UK parcel industry and batteries can no longer be sent with acid separately. I remember when the ban was introduced because because it happened very quickly and caught a lot of folk out, I'm surprised you did and wonder if it's old shelf stock.
I bought a battery from Halfords at the weekend for the GS500, tiny little thing but it still came in the same sized box as if it would have had acid, I bet they've been through their stock and filled them to comply.