They need to be attached to the studs or blocking behind the tile/drywall. Typically blocking is planned for the pipes to be attached to.

The dilemma is that its an existing stud wall and the studs don't line up with the shelf flanges.

you can put in extra pieces of wood to fit between the studs where the shelves will go. that's what I did w/mine. You'll have to redo the wall board, but not a big deal.

can't really find a pic, but something like this. see the horizontal studs? you would place those between the wall studs, securely, exactly where your shelf will go. then the lag bolts will get attached to these horizontal pieces

or, you can use something like this

this might be easier, but it will cost more

you attach the span portion directly into two studs. these come in diff lengths w/varying spaced holes.