All water will pick up CO2 with a resulting drop in pH, and very low TDS water like distilled or good quality RO can easily turn acidic given enough time and the typical LDPE plastic containers that allow CO2 to pass through. If you wait long enough you can get distilled water to register in the 5.something range.
Yes, some meters have increasingly difficult time reading accurately as TDS approaches zero so I wouldn't use the numbers literally but they still make the point that distilled water containers rarely register as 7.0 pH for long if ever...
Cheers!