S-line is actually more like the m-sport pack on the 3 series. So you can get an A4 2.0T prestige s-line, which adds things like black headliner, black window surrounds / grill , s-line seats with alcantera , s-line 19" rims etc. same suspension as the normal sport pack though (i know this becuase I have a sport pack A4 right now)
Now there are the S series cars which they seem to call their midlevel cars but only on some models. with audi, they call the S4 the S4 and not say the A4 3.0 T or something (and in other countries they do have an A4 3.0T which has like 270hp unlike in the US).
it does sort of make marketing sense since BMW uses numerical model designations and audi doesnt, it sort of just looks nicer. I mean A4 2.0 , A4 3.2 , A4 3.0T (270hp) and A4 (3.0T 333 hp if we were going with this scheme) is probably just harder to market than S4 or A4 (at lesat in america where theres really only one model for each).
Or their scheme could just be nonsensical. the Q7 has 2 3.0T variants in america, the 272 and 333 hp. but its not an SQ7 or something.
That said every company has weird marketing inconsistancies. BMW seems to be changing some of them with the 4 series (i mean if coupes really are even numbers like z4, 6 series etc) . but they are adding even more with weird stuff like all the grand coupes . I mean the newest 740i and 640i have the same N55 engine as say the 335i but it gets a 40i. and they rated them higher, without really doing much becuase its underrated in the 335i. and i guess 735i doesn't look good?
I mean, the 2006 E90 325i had a n52 that made 215 hp. the 2007 328i had basically the same engine with absolutely no difference and they called it 230hp and a 328i.
blame it on marketing and product management.