Quote Originally Posted by Domehead View Post

"In Scotland crests have always had a very much less important position than in England. There has been little if any continuity with regard to them, and instances of changes for which caprice would appear to be the only reason are met with in the cases of a large proportion of the chief families in that kingdom. To such a widespread extent has the permissive character been allowed to the crest, that many cases will be found in which each successive matriculation for the head of the house, or for a cadet, has produced a change in the crest, and instances are to be found where the different crests are the only existing differences in the achievements of a number of cadets of the same family. At the present time, little if any objection is ever made to an entire and radical change in the crest--if this is wished at the time of a rematriculation--and as far as I can gather such changes appear to have always been permitted." Ch.21, A Complete Guide to Heraldry, Arthur Charles Fox-Davies, 1909
When quoting Fox-Davies it should be remembered that, despite all of his writings on the subject, he was never a herald.

In Scotland crests assume the status of transmissible badges (hence their use within a strap and buckle by retainers and followers) which explains why various branches of the same family will have dissimilar crests.