Can someone explain?

Well, there is likely a few reasons for this. Xerath is one of those "long range poke poke poke and win" champions that people often do as well as you do on in aram. Thats like getting a score like that on Lux or Ziggs. People do it way often, so you have to do better to get an S on Xerath rather than Malzahar who is needing to get up close most of the time and will often times die more often.

Also damage to champions is part of it, did the Malzahar have a lot more damage to champions? They also have more kills, rather than assists, which is harder to get in ARAM.....

Honestly it just seems like they barely got the S- and you barely did not.