I think they're referring to requirements files. I've seen the same behavior - on day 1, pip installs packages A and B, then on day 2 when someone else modified the requirements file it installs C and reinstalls B even though B hasn't changed.
This one I've seen when the dependency doesn't specify an exact version and you included "--upgrade".
There's a second case that I think was fixed in pip 21 or 22, where two transitive dependencies overlap - A and B depend on C, but with different version ranges. If A allows a newer version of C than B allows, C can get installed twice.
This one I've seen when the dependency doesn't specify an exact version and you included "--upgrade".
There's a second case that I think was fixed in pip 21 or 22, where two transitive dependencies overlap - A and B depend on C, but with different version ranges. If A allows a newer version of C than B allows, C can get installed twice.