Lu sur "Inside racing technology" une explication qui me semble tout à fait plausible sur le pourquoi du comment une trajectoire qui serait bonne sur le sec ne le serait plus (forcément) sur le mouillé :
The Myth of Off-Line in the Rain
We've probably all seen experienced drivers steer their cars off the normal line when cornering on wet pavement. The usual explanation is the normal line is slower in the wet because of oil and old rubber.
If there actually is more grip offline it's because the surface features are higher and more pointy than the more worn features on the normal line, providing higher deformation friction. This phenomenon is exaggerated by high-hysteresis compounds in racing rain tires. When dry the "line" is the fast way around that turn because it's shorter and has the biggest radius.
I've talked to several race drivers about this phenomenon and a couple of them pointed out that offline in the wet worked on dedicated racetracks and not on street courses. The reason for that is there is no "line" on a street course; the aggregate is worn everywhere by daily street traffic. But on a dedicated road course, 99% of the cars on the course are working the racing line and wearing the aggregate in the process.
L'adhérence sur sol sec vient à la fois du phénomène d'adhésion pur et simple mais aussi de l'effet d'engrènement mécanique dû à la rugosité du revêtement. Sur le mouillé, le phénomène "adhésif" perd de son importance relative, ce qui peut amener à privilégier des portions de la piste où la rugosité est plus importante, c'est-à-dire les moins usées.