Lots of reasons, mostly for marketing. Would you buy an '05 S540 V12 to replace your '95 S600 V12? Doesn't S63 catch the eye of those familiar with the legendary 300SEL 6.3 more than a displacement appropriate S62? Recent practice is to uplift force aspirated engine badges to comparable naturally aspirated displacement with fudging to appealing numbers. I spent a couple of minutes at this site - https://www.mbusa.com/mercedes/vehicles/model/class-CLA/model-CLA45C4#!layout=/vehicles/model/specs&class=CLA&model=CLA45C4&waypoint=model-specs - and I don't see an engine displacement figure which is probably 2-liters. I think they're playing fast and loose with the AMG sub-brand.

Sixto
83 300SD, rounds to 3.0 liters
98 E320 wagon, rounds to 3.2 liters