Can anyone explain why the number of sling kills is higher than apex kills?

Apex Kills: BR-only - kills in "the Apex Games" (anything with the "Apex" prefix will only be the "core" BR modes - Trios, Duos, Ranked, etc)

Sling Kills - includes Mixtape kills

i think it's already quite high given that it only actually tracks kills with the weapon you have in the sling. 1700ish kills in any game mode with your sling weapon. 1300 kills with any weapon in BR. Those feel comparable.