I've found a few sources saying that GH can be reduced by plants, peat, and driftwood
[1] [2]. There are others, you can just search for them.
Plants uptake Ca and Mg as they grow, but it seems like they typically uptake a small amount (from source 1). However, source 1 continued to feed the tank food during their test. This probably introduced more calcium and magnesium to the tank. If your tank is planted, unstocked (and unfed), and you haven't made significant water changes on it since setting it up, it's possible the plants have been able to put a dent in the GH.
Peat and driftwood seem to do exactly what I suggested earlier (source 1 and 2 mention this). In the process of releasing tannins, peat and driftwood pull Ca and Mg ions out of the solution. This basically works the same as your standard water softener except water softeners replace the Mg and Ca with Na while peat and driftwood replace Mg and Ca with organic compound ions. Again, it seems like it typically can't cause massive change in hardness, but it sounds like it has more of an effect than just plants. Interestingly, it sounds like peat and driftwood don't reduce the total dissolved solids (TDS) because they're replacing one type of ion with a different type. GH tests only test for Mg2+ and Ca2+ and thus, they can't measure the organic ions released by the peat and wood.
The wood in the tank in combination with live plants, the lack of fish food being added (I'm assuming), and small water changes could be responsible for the discrepancy in hardness between tap and tank. Once you add fish, start doing larger water changes, and start feeding more I would expect the tank hardness to revert back to the tap hardness.
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