DI's do alot of stuff that is out of regs IOT look better.
Not supposed to starch cammies or tailor khaki shirts to be formfitting, shaved heads are eccentric, not supposed to use chevrons or other pinning devices on the back of your shirt to hold the creases, not supposed to "tack" your pockets down or use glue on any part of your uniform, etc...
Not everyone does these obviously.
I have heard stories of Marines cutting their sleeves to make the rolls look tighter (or of Marines whose arms were too big to properly roll the sleeves), but I have yet to see it firsthand. Seems like it is another uniform rumor along the lines of putting fishing line in your creases to make them stand out (heard, but never seen).
Good sleeve rolling just takes a little experience and some patience, don't stress out about it...