Sounds like a bunch of bullshit to me

marijuana isn't well studied (blame the overzealous drug enforcement agency), but it shares the properties of stimulant, depressant, and hallucinogen. i'm sure there's no definite answer to your question, but if i had to guess it's because pot's a sensitizer. it amplifies physical sensations, so something like filing papers becomes far more interesting simply because textures and movement feel...odd. it also allows your mind to wander more easily, meaning you can "zone out" while still doing menial tasks.

if you really want to concentrate, however, i recommend adderall or vyvanse. they're amphetamines, but not like cocaine or meth.

It makes me scattered brained if anything.