Because the EEG electodes 10-20 template is in MNI space and is fitting to MNI space surface
I thought you were importing the individual MRI of your subjects... I didn't understand correctly then.
If you are using one of the MNI template in Brainstorm, you don't have anything to do. The 10-20 template is already aligned on these templates (at least for Colin27 and ICMB152).
Note that it is probably vain to spend a lot of time trying to be very precise for the EEG source modelling if you are using anatomy templates...