To make things worse (or better), æ, œ, and ß have evolved from ligatures to becoming letters in their own right (a choice that may depend on the language), while fi stayed a ligature. So 'fieß', if there was such a word in German, would count as 4 letters.

Also, talking about 'glyphs', one has to clearly separate between what Unicode encodes and what a given font uses; in the context of fonts, 'glyphs' are also called 'glyfs' or 'outlines', and any OTF my choose to render any given string with any number of glyfs/outlines, including using a visual ligature to display 'f+i' = fi but doing so by nudging the outlines for f and i closely together.