Baeyer–Villiger oxidation

In the Baeyer–Villiger oxidation the migratory aptitude (according to what I've been taught) is listed as the most substituted group > least substituted group.

I have two question : Why is this the pattern of migration ? Wouldn't it be better to leave the most substituted group behind to allow for the stabilisation of the positive charge formed when the group migrates to the oxygen ?

Also, why on wikipedia is Hydrogen listed as a better migratory group than tertiary alkyl ? Is this due to sterics ?

There is an explanation here.

http://classes.yale.edu/02-03/chem125a/125webSpring/BaeyerVilliger/migratoryaptitude.htm

I may come across as a bit slow here but I cannot see where or how a positive charge is being developed in the molecule.