Examples of densities of some pool chemicals are in this thread where the second number (bulk) is the relevant one. You can see that densities vary from 0.837 for calcium chloride dihydrate to 1.44 for dry acid. The one ounce weight to one ounce (1/8th cup) volume approximation is only correct when the density is 0.959 g/ml.

Sorry for the bad formatting...the "CODE" formatting on this forum no longer works (that's used to force a fixed space font which is good for lining up columns).