Perhaps Sandy will come on with an answer specific to the Macleans, but in the case of most others it is/was often how it was written by someone other than the holder of the name. So Macdonald/MacDonald/McDonald/M.Donald referring to the same individual in four different documents. For some reason several generations ago a chief of the Macintoshes himself wrote his name as MacKintosh and it has remained that for the chiefly line ever since.

So it's not accurate, T-Bone, to say that all other "Mac" clan names use an upper case letter following: Macgregor is often found as MacGregor, Macgillivray (and McGillivray) more often than MacGillivray, Mackenzie more often than MacKenzie, Maclachlan more often than MacLachlan, for example.

In Scotland, that is. I understand that Ellis Island contributed a lot to name-change in the US.