It's not too hard, while they share some mechanics, the underlying use-cases and requirements are very different.

_______ Optical character recognition:

1. You have a set of predefined patterns of interest which are well-known.

2. You're trying your best to find all occurrences of those patterns. If a letter appears only once, you still need to detect it.

3. You don't care much about visual similarity within a category. The letter "B" written in extremely different fonts is the same letter.

4. You care strongly about the boundaries between categories. For example, "B+" must resolve to two known characters in sequence.

5. You want to keep details of exactly where something was found, or at the least in what order they were found. You're creating a layer of new details, which may be added to the artifact.

_______ "Glyph compression":

1. You don't have a predefined set of patterns, the algorithm is probably trying to dynamically guess at patterns which are sufficiently similar and frequent.

2. Your aren't trying to find all occurrences, only sufficiently similar and common ones, to maximize compression. If a letter appears only once, it can be ignored.

3. You do care strongly about visual similarity within a category, you don't want to mix-n-match fonts.

4. You don't care about clear category lines, if "B+" becomes its own glyph, that's no problem.

5. You're discarding detail from the artifact, to make it smaller.