In theory, cuneiform glyphs are numbered to distinguish homophones: if there are two common glyphs pronounced
/u/, one will be named U₁ and the other will be named U₂. If a glyph has multiple readings, the number is attached to a particular reading, so the glyph BI₁ is also known as PE₂, PI₂, BE₂, and SA₁₈, among others.
So far so good. But sometimes, the numbering seems to carry over from one reading to another. For example, the glyph GU₁₀ can also be read as
/ŋu/, and there are no other common glyphs with that reading. But its alternate name is ŊU₁₀, not *ŊU₁—in fact, the name *ŊU₁ doesn't seem to be associated with anything at all.
Why does this happen?