Why Do Some Scots Say 'Mony A Mickle Maks A Muckle'?

The English translation of this is basically many little things make a lot of things.
It is very old Scots and is used mainly on the East Coast of Scotland and the outlying islands such as Shetland and Orkney, but it is not in common use.
Historically it was used by George Washington in 1793, when he was writing about aggragates.
Some (English) lexicographers feel that it is actually wrong and he was confused about the words mickle and muckle.  But muckle is still used on old Scots vernacular today and is prevalent in some Scottish dialects.
So it does make sense when viewed from the Scottish perspective: but the English seem to think its all a bit muddled !