A small ABV disagreement between this site and a different piece of brewing software almost always comes down to one of three things — and none of them means either tool is doing the arithmetic wrong.
Formula choice, and it matters more than people expect
(OG - FG) x 131.25 and (76.08 x (OG - FG) / (1.775 - OG)) x (FG / 0.794) are both legitimate, widely-published ABV formulas, and they agree closely at normal beer strength but diverge as OG climbs. At OG 1.090 to FG 1.020, the simple formula gives 9.19% ABV; the advanced formula gives 9.99% ABV — a gap of nearly a full percentage point. Check this exact pair. At OG 1.110 to FG 1.024 (barleywine territory) the gap widens further still, to roughly 1.4 percentage points. If your recipe software defaults to one formula and this site's calculator defaults showing both, comparing "the number" between them without checking which formula each is actually reporting is comparing two different calculations, not the same one.
Real vs. apparent gravity readings feeding the calculation
Software that pulls OG or FG from a refractometer reading needs that reading corrected for alcohol's effect on refractive index before it's usable as an SG input — a refractometer reading fermented beer without correction overstates the true gravity substantially, and any ABV calculated from that uncorrected number will be wrong regardless of which ABV formula is applied afterward. See refractometer correction, explained — if your software applies this correction automatically and you're entering an already-hydrometer-read SG into this site's calculator, that mismatch alone can produce a real difference.
Rounding and display precision
Some software rounds intermediate values before the final ABV calculation (rounding OG and FG to three decimal places is standard, but some tools additionally round attenuation or extract figures mid-calculation), which introduces small compounding differences from a tool that carries full precision through to the final display and rounds only once, at the end — this site's calc core rounds only at the display boundary, never mid-calculation, specifically to avoid this class of error.
How big a gap is actually worth chasing
At normal beer strengths (roughly 1.040-1.065 OG), expect any two reputable calculators to agree within a couple tenths of a percent — a bigger gap than that at normal strength suggests one tool is applying a correction or rounding step the other isn't, worth tracking down rather than ignoring. At higher gravities, expect the gap to widen naturally, and lean toward whichever formula your software documents as "advanced" or "high-gravity accurate" for anything above roughly 1.075 OG — see common OG/FG combinations and their ABV for how the two formulas compare across a wider spread of starting gravities.