I'm fairly certain you're referring to this thing: http://insideracingtechnology.com/tirebkexerpt1.htm EDIT: So yeah, seeing how tyres generate grip by three main means (adhesion, deformation and tearing/wear) and any water between the tyre and the track eats away or prevents the formation of adhesive forces, the means of developing grip in the wet are left to deformation and tearing/wear. The racing groove that is worn smooth offers less grip than the rougher off-line in these conditions, only supported by the fact that street circuits which are more worn overall don't seem to be as affected. You've probably heard how after resurfacing tracks tend to be a lot faster, for example during practice season. For the life of me I can't find the source right now, but I remember reading that Laguna Seca was something along the lines of 2-3 seconds faster than normal after it's last resurfacing, but as the season draw nearer it was back closer to old speeds, the initial roughness of the surface that gave more deformation and tearing/wear grip had been smoothened out somewhat by the the cars driving around there. It might well be that overall the off-line does have better grip in normal conditions too (except obviously when the racing groove is properly rubbered in) but it's still faster because the line is just that much better and when all three grip producing mechanics are available the difference isn't as great as it is in the wet. EDIT2: Not saying this is conclusive, but it does make a helluva lot of sense and the science behind it seems better than what your average driver or chief mechanic tends to give you.
Then add clean / dirty road surface to that, which changes things a bit on dry, probably has some effect for wet too, but not quite sure how that works. Anyway outside of racing line tends to be dirty, those marbles for example, then dirt etc. which acts kind of tiny ball bearings or just material that has less friction than perfect rubber - asphalt contact. Sometimes when car with good aero goes off the line you can see huge dust cloud coming off from the track, also tires change color from all that dust.
