In this particular use (for bolognese or pescatora, amatriciana, etc.) "a/al/alla" means "in the style of", BUT when it is pasta al ragù (al pesto, al pomodoro, etc., i.e. other main ingredients) the "a/al/alla" means "with".

And we have also our own exceptions, one says "Pappardelle sul cinghiale" (pappardelle is yet another type of pasta), literally "pappardelle on the hog", BUT you don't have the "a" in "spaghetti aglio e olio (e peperoncino)".

As a side note a "ragù" has nothing to do with the (usually horrible) tomato sauce that is used for "spaghetti bolognese" outside Italy.