Assuming you've presented all the informaton about what's going into your tank, the only possible answer is that there's something in your substrate that is "fooling" the test kit into giving you a much higher CA reading than you actually have. At least that's the only possibility I can think of. To make sure I've understood everything, tell me if I'm missing anything:
1.You test freshly mixed salt with your test kit, using the water that's also in your tank. The test result is a little low, but more or less normal.
2. You add this to your tank, and add no other substance except for superbuffer which is basically sodium carbonate/bicarbonate mix. You measure an impossibly high Ca level, unless your ph is so low that nothing would survive in your tank.
3. You have a relatively new tank with sand that you're not convinced is appropriate.
4. Since it's impossible to have that high a Ca level, the test result must be wrong; this also explains the extreme jump in level from the SAME SALTWATER that was in an external mixing tub. You're fairly confident of the test kit, because it reads something reasonable in the mixing tub. So something in the tank, my guess is the sand, is skewing your test result.
Have I misunderstood anything? Here's the bad news...if you want to replace the sand in your tank, you probably need to get all your livestock out, all the rock, and drain the tank, scoop out the sand, replace it, start over with entirely new water, toss a couple of pieces of live rock in to help seed the new sand bed, maybe some live sand from a friend's tank, let it settle for several days, and rebuild your reef. Sorry about that!
In your case I would suggest going to a local fish store and getting some "pure caribbean" aragonitic sand, or ordering it from the place in FL that sells it. It's more expensive than southdown, but it will clear up faster and you'll have confidence that it's the right stuff for a reef tank.