dddd

The cause I would expect would be that the difference in tooth count between the small and middle ring is as small or smaller than the difference in tooth count between the middle and big ring.

The chainring sizes have a huge impact on how robust of a shift that the rider can get while shifting up to the middle ring and down from the middle ring as well.

Shifting performance to and from the middle ring is best with half-step style triples, but short of that one should try to keep the difference in tooth count between the small and middle ring AT LEAST half of the total range/difference.

Note also that newer triple front derailers have "step" features formed into their inner cage plate which correspond to very specific tooth-count differences between the middle and big ring.
Violating the intended tooth count step can make for a failure to shift up to the middle ring with any authority at all.
Remember here that it's the big ring that sets the derailer's height, so the tooth count difference with modern front derailers references only from the big ring down to the middle ring.
This applies to both road and mountain triple derailers from roughly the last 20 years.