This happened to me back in fall 1996. The store had sold me a pair of end of season Fischer RC4s with a race plate.
The store owner placed his jig on the plate, not on the ski edge which was too far away to be gripped by the jig's jaws. The ends of the race plate had been cut by the manufacturer on a diagonal for artistic reasons and the jig could grip one side of the race plate better than the other, so the net result was that the jig was not sitting securely on the race plate.
Another problem was that there were sleaves on the jig to position the posi-drive drill bit and these sleaves had been worn over the many years that this jig had been used, so when the drill bit was placed in the sleave, there was some looseness that caused the hole to be slightly offset. This looseness in the sleaves also allowed the shaft of the drill bit to be off vertical, further offsetting the hole from the desired position.
Another problem was that the race plate was not perfectly centered on the axis of the ski, so the jig was referencing to a surface already in error.
My bindings were off in the heel by as much as 1.5 mm.
Earlier that year I had gone 136 kph on the speed track at Les Acrs using my older GS skis, and I told the store owner that I did not want to go down the speed track using these Fischer skis with their 1.5 mm binding error.
The store owner did not have another pair of previous season RC4s but I accepted a more expensive pair of next season Fischer Revolution Race RCR by paying the difference in price.
I never went back to the speed track, but I did turn in my best ever handcapped score in a GS on the RCR replacement skis.
I insisted, and the store agreed to let me personally mount the bindings on the RCR skis with the store's jig, whereby I noticed all the pitfalls that I noted above.
Since that day, I have always mounted the bindings on my skis using tools such as paper templates and factory pre-drilled race plates as guides. It may take me up to 2 hours to do it, but I always end up with accuracy that I am comfortable with.
Modern day skis often come with mounting plates with dimples so that it is next to impossible to set the hole in the wrong location.