If you were happy with the old skates position, put them back on and mark where the actual line of the blade is on the sole. You might have to make a mark on the edge of the sole at the front and on the heel if you can't mark the sole easily. then just line up the new blades (not the mounting plates) with the marks. Use the elongated screw holes (make the hole at the mid point of the elongated hole so that you can adjust them either way) to mount at first, so that you can adjust the position when you try them.
I expect that they just happened to find a few holes that fitted, rather than tried to line them up.
