Recall the standard stem for Latin comparatives is -issimus. However, most student of Latin are familiar with a variety of alternative, irregular forms. For instance:
Bonus, melior, optimus
Malus, peior, pessimus
Magnus, maior, maximus
These are all more archaic forms of the superlatives. (Hence there appearance in very basic, common adverbs, which we can predict would be more resistant to phonological change due to frequency of use.)
Furthermore, A&G note that certain superlative adjectives are derived from their comparative forms, not from their positives. They aren’t explicit about how this works, but the example they offer is extrēmus, which might go exterior –> *exterīmus –> extrēmus.
A&G compare this to the derivative development of ‘childish’ superlatives like the English furtherer and furtherest. Again, this is all a little mysterious to me, as a non-phonologist. If anyone has thoughts, I would love to hear them.