Here is a little more detail on what is occurring:

>>> from unicodedata import *
>>> c = 'µ'
>>> for s in (c, c.lower(), c.upper(), c.lower().upper(), c.upper().lower()):
	print(s, ord(s), name(s), category(s))
	
µ 181 MICRO SIGN Ll
µ 181 MICRO SIGN Ll
Μ 924 GREEK CAPITAL LETTER MU Lu
Μ 924 GREEK CAPITAL LETTER MU Lu
μ 956 GREEK SMALL LETTER MU Ll