etymology - Why are not "infamous" and "inflammable" the opposite of "famous" and "flammable"?

The New Oxford American Dictionary reports the following definitions for in-:

in- 1
prefix
1. (added to adjectives) not: inanimate | intolerant.
2. (added to nouns) without; lacking: inadvertence | inappreciation.

in- 2
prefix
in; into; toward; within: induce | influx | inborn.

Looking at the origin of infamous, and inflammable, I read the following:

infamous
ORIGIN late Middle English: from medieval Latin infamosus, from Latin infamis (based on fama 'fame').

inflammable
ORIGIN early 17th century: from French, or from Latin inflammare (see inflame).

In both the cases, the words are not built adding the prefix in- to existing words.