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Increase in GH

The following isn't a problem, just a puzzle -- to me, at least.

I have a 30 gal. tank, quite a few plants, and 35 fish.  Normally, right
after I do a 50% weekly water change, I do tests for pH, KH, CO2 (derived
from chart), NO3, K, GH, Ca, Mg (by subtraction), Fe, and PO4.  Then I make
any adjustments that might seem indicated, e.g. add 1/4 tsp baking soda
(occasionally), 1/2 tsp MgSO4 (rarely), 15 ml TMG (always).

This past weekend, I decided to do all the tests *before* making the water
change to see how levels had changed over the previous 7 days.  No surprises
in most tests: NO3 up from 5.5 to 13.2; K down from 35 to 20; Fe down from
0.04 to 0.01; PO4 up from 0.08 to 0.15.

But here's one I don't understand: the GH went up from 120 to 140 ppm.  I
thought I might have miscounted the drops and repeated the test (Hach kit).
Same result.  I hadn't added any Ca or Mg during the week.  Could someone
tell me what else could have raised the GH?  Thanks.
John T. Fitch
Web Page: www.fitchfamily.com