Because GEX is an estimate built on assumptions, different providers legitimately arrive at different numbers for the same market. The disagreements come from a handful of modeling choices.
What actually varies
The dealer-positioning sign. The whole calculation hinges on assuming which side dealers are on; providers use different rules for signing flow, and small sign differences move the flip. Which expirations. Some include only near-dated options, others the whole surface — and since 0DTE and monthly gamma behave differently, that reshapes the profile. Data and timing. Snapshot time, data source, and whether they use live or end-of-day open interest all shift the result. Modeling. How gamma itself is computed (volatility inputs, spot vs strike weighting) differs.
The disagreement isn't error — it's different honest choices about an unobservable quantity. Pick one method and stay consistent.
Why it doesn't break the framework
The direction of the read usually agrees even when the exact level doesn't: providers tend to concur on whether the market is broadly long or short gamma, and roughly where the big walls sit. The precise flip price wobbles; the regime call is more robust. Trade the regime with confidence and the exact level with a margin of error.
The practical rule
Don't average two providers or chase whichever number suits your bias — pick one consistent source and learn how it behaves. A slightly “wrong” level you understand and use consistently beats a “truer” one you switch to only when convenient. NoVo computes its map one consistent way for exactly this reason: internal consistency matters more than matching someone else's dashboard.