Check your PH test kit - if you are really swinging 2.0, then your tank could crash any second. Since it hasn't, it could be the kit. Do you have PH pen, or meter? PH is on a Logarithmic scale and each .1 is twice as potent as the number before - 8.0 is 10x as basic as 7.0... and 9.0 is 100x more basic than 7.0. If you are swinging 2.0, then we are talking 100x.
If you have a super high, or want to have a super high, calcium consuming bio load, then look into a different supplementation method. I cannot evaporate enough water to add in enough saturated kalk, or 2 part, into my SPS systems - I imagine that the clams alone use more than what i can add.
Even though they are all tied together, they don't always get used in balance. In systems that I have had that are heavy with clams, they use the calcium up faster than the alk, so about once a month I have to use some calcium chloride to bump from 360-380 up to about 420-440 again. In my case, I am melting coral skeletons. Maybe if I was melting clam shells, it would get used in the same balance that it is being melted.
There is tons to read on this, but here are the basics.... if either calcium or alk is way too high, the other will be lower. They will somewhat naturally balance if you have a stable/good amount of magnesium. If you add them with a balanced method (kalk, or Calcium Reactor), then it is hard for them to get too out of whack too quickly if your mag is good. If you add independently (manual or with 2 part), then you need to check more often and have some good fail-safes. The ocean mostly supplies calcium and alk from the substrate where bacteria produces acid to lower the PH and causes the aragonite matter below the surface to dissolve - this is the same as in a calcium reactor, but it uses depth, pressure and bacteria to lower PH instead of CO2 from a welding shop.
Check these out:
http://www.reefkeeping.com/issues/2005-01/rhf/
http://www.reefkeeping.com/issues/2002-04/rhf/feature/
http://www.advancedaquarist.com/2003/2/chemistry