When two markets that look identical are not the same contract

Matching markets across venues by title is easy and almost right, which is the worst possible combination. Two contracts with near-identical titles can settle differently, and when they do an arbitrage position loses on both legs at once.

Different resolution sources

The most common divergence. Both venues ask who wins a race; one resolves on the official governing body, the other on a named data provider. Ninety-nine times out of a hundred they agree. The hundredth is a disqualification, a correction, or a provisional result that changes — and that is the one you are exposed to.

Worse, the source is usually written in prose rather than a structured field: "Outcome sourced from MotoGP", "according to the Associated Press", "resolution source: NOAA". A matcher that only looks for one phrasing silently records "unknown source" across half the universe, and an unknown that every pair carries tells you nothing about any of them.

Different close times

Two markets on the same event can stop trading hours apart. Whether that matters depends entirely on the horizon: eleven hours apart on a contract settling in eighty-one days is immaterial, while eleven hours apart on something settling tomorrow means one leg is exposed to news the other is not.

This is why a fixed threshold — "more than six hours apart is a problem" — is the wrong tool. The gap has to be weighed against the time remaining.

Different scope

The subtlest case: both markets name the same event but define it differently at the edges. Does "wins the election" include a runoff? Does "above 3%" mean the first print or the revised figure? Does the market void if the event is cancelled, or resolve NO? Two venues can answer those differently while using the same twelve-word title.

What to do about it

Grade the match rather than accepting or rejecting it. A pair where every machine-checkable part of both rule texts agrees is a different proposition from one where the sources could not be parsed, and both differ from one where the close times are a day apart on a week-long horizon. Show the differences to whoever is about to size the trade, and let a difference you cannot explain be a reason to pass.