I remember asking about this years ago and no one could figure out why then either.
I have a thirtieth level set piece of armor I just crafted for an alt. I have two 30th level armor glyphs in my possession, both crafted by me.
They are a greater glyph of magicka and a moderate glyph of magicka. Both are for a minimum armor level of 30, yet only the moderate glyph appears to be available to enchant the armor. And yes, both glyphs are in my inventory.
This is the glyph that isn't showing up as usable. I am assuming it's not a bug, and there is some reason it's not usable on the armor I crafted. Is it only for non-crafted items at that level or something?