shorter arc levers

they are not short but fastened further inwards 

Jett runs renthal fat bars with the perch mounted further toward the center so a finger or 2 of the lever reaches the grip.

ASV makes a shorty lever, but I don’t know about ARC

ARC does now offer shorty (and also skinny) levers to the public. 

I'd assume that the extra leverage is why Jett would choose to move the standard levers inward rather than use a shorty lever.

There was an interview with Jett's Dad or somebody that knew about the stuff they did , and whoever it was talked about things they did when they didn't have a lot of money. Keeping the levers more inside like that was one of the things they did to try and prevent levers from breaking in small crashes. And it's stuck with him and Hunter.

 I'm sure there are more reasons than that, but that was what stuck in my memory from the interview.  

Not sure why anyone would run a shorty. Run it further in and keep the leverage for easier pull.

No handlebar room? Can you elaborate?

A lot of the newer bikes have so much on the bars already that there is no room to slide the levers in.  If You put handguards on then there is no room. I put some Flo levers ( I know , not ARC and ARC do not need as much space as the FLO) on a current KX250f for a customer along with Acerbis Uniko handguards and the way the levers fold there was very little freedom of how I could adjust the levers and not make the handguards end up in a position that would no longer make sense . Its very tight on modern 4 strokes.  I've had similar issues with room on 65's and 85's. 

That makes sense for sure if running handguards. I was able to run my levers in pretty far on my Honda, but no guards.