I wouldn't expect those heads to hit the same place........You are changing FOC, spine and total arrow weight pretty significantly. In other words you have changed the tune of your set-up when you go to 100gr heads.
Did you try a 100gr field point to see if it hits consistently with your 100gr Stingers?
If THEY hit together but seperate from the 125's your "problem" is no problem at all.........just a change in arrow to bow tune.

Using your 2315 and a few guesses about your bow as the example with say at 29" total shaft length a 100gr head is listed as a possible choice but as being hard or stiff spine wise....... if you are shooting vanes Vs. feathers it will magnify the stiffness.

When you plug a 125gr head into the equation it pops up a 2315 as "Popular" choice and a medium spine.

You can't realistically compare the 2 or expect them to hit the same place on your target.
Like I said though, if 100gr field points hit away from your 100gr stingers then you do have some sort of issue, minor or major.