Shipping by weight calculated incorrectly on order

Hello! 

I have a shipping by weight problem. I originally had shipping set to standard 1st class Royal Mail, small parcel (up to 1kg). However, A recent customer order went over the weight limit so, I had to cover the additional cost. I've now changed to ship by weight but, the shipping on the first order since changing has been over charged. The product ordered weighs 350 gm and the customer ordered 2 so, that should be a total weight of 700 gms and be within the limit for my 'up to 1kg' shipping option. However, the customer has been charged at the next level up. I can't work out why this has happened. Can anyone help, please?