The only reason I can think of there would be a wire all the way up that stud would be it's the begining of the line and the wire would be run up the stud and through the top plate. You should be able to see it in the attic if that's the case.
To be safe just use 1-1/4 screws that way your screws will be no longer then the sheetrock screws.
I strongly suggest you use any brackets other then those. There not very strong and ugly to look at. They sell corbel brackets at Lowes and Home Depot that will look far better and be stronger.
I've removed dozen of those things from peoples garages were they tryed to DIY some shelves and they just bent.
Also there so wide your going to have to be dead on to hit the stud with the top side screws.
One's like these have a key hole slot on the back side with a metal plate and only need two screws to hold in place.