1) You could be losing water and constantly adding more, as Chem_Geek pointed out.
I lose water to evaporation, which should not effect chemical levels. I lose some water when I backwash. I lose some when a heavy rain raises the water level above the drain. I am pretty sure there is no leak.

2) Your reagents are old and not effective and need to be replaced
3) You are running the test incorrectly
4) You are testing with strips which are VERY difficult to infer exact values from (I find them worthless for CH and TA)
I think I am running the test correctly, with good reagents, because the pool store, which is also doing the drop test, gets the same results. How long do the reagents stay good?

5) You simply aren't using enough Cal-Hypo and calcium isn't effectively increasing.
My guess it that this is what is happening. I used about 50 lbs in my 22,000 gallon pool over the summer. Does anyone know how much calcium is in a pound of 68% cal hypo?

6) You are wasting your money on the 48% stuff instead of the 62-68%. WalMart pushes that junk instead of the good stuff. It only SEEMS cheaper.
This is the stuff I am using. It is 68%. Amazon delivers it, and is the cheapest place I have found.
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
It ends up being a little cheaper than liquid chlorine bleach, and easier to use.