@cacman I believe I see your problem with your 2. something clarification. There is a specific technique you must use to zero out the slot caliper to measure DFT. Here's some pictures to help.
So begin by sliding the slot all the way up to the the back of the fin, then slide the moving measurement head( with Display) all the way up to the very back of the fin also, then ZERO it out as in the first picture. Then move the measurement head to the back of the ski as in the second picture.
That is the slot caliper technique to have it display a DFT measurement that is correct. As others have noted, your ski, caliper and technique will all cause some variation in your results. Find a method that you can use to get repeatable results. Cheers.