It is likely to be one of two things - possibly both. One is the exact measurement of your pupil centres not being accurate, the other is that the specs need to be seated on your head as everyone has different configurations of ears, nose bridge and the lumpy bits behind your ears. Even slightly off will give you double vision with specs that are of the right prescription. This is what the dispensing optician does at the shop - they use that hot air machine to soften the plastic and shape it to you, and this is something that is impossible to do online.
You could try sweet-talking a local opticians but even if they agree to handle it I bet they charge you. The likes of Specsavers will just say they can't handle 'other' specs.