The same question is priced on more than one market, and the prices differ. That difference is the most useful number in the category.

Two prices, not one

A price on a prediction market is the output of a specific population. The people trading a given contract on a given venue have a particular composition: where they are, what they read, what else they trade, how much attention they pay to this question rather than the next one. Another venue, pricing the same event, has a different population.

So when the two prices differ, the first thing to understand is that nothing is malfunctioning. Two instruments have measured the same thing and returned different readings, and the difference tells you something about the instruments as well as the thing.

Four reasons a gap exists

The contracts may not be identical. Two venues may ask what looks like the same question and resolve on different criteria: a different source of truth, a different cutoff date, a different treatment of edge cases. A gap between them is then a gap between two different questions, and the prices are both correct about their own rule.

One book may be thin. A price produced by a handful of participants can sit at the wrong level indefinitely because nobody has arrived to correct it. The gap is then an artefact of an absence, not a disagreement between two considered views.

One population may know something the other does not. A development that has reached one venue's participants and not the other's, a reading of the question that one crowd has seen and one has missed. This is the most interesting case and the least common.

Or one price may simply be stale. It has not traded in an hour while the other traded four minutes ago, and the gap is the interval.

Why the order matters

Those four explanations are not equally likely and they are not equally cheap to check. The contract terms can be read in seconds. The depth of a book is visible on the venue. The time since the last trade is printed beside the price. Each check, if it explains the gap, makes the next one unnecessary.

A reader who jumps to "one market is wrong" without passing through the cheaper explanations has not done analysis. They have expressed a preference. The gap they are looking at will, more often than not, dissolve on inspection into a difference in terms or a thin book, and the reader who has already acted on it has paid for the difference.

What the gap is worth

When the cheap explanations have been ruled out and a gap remains, it is worth more than either price alone. One price is a reading. Two prices, with their terms alongside, are information: a visible disagreement between two paying populations about a precisely specified event.

That is the number a single-venue reader never sees. And it is nearly impossible to see by hand. Locating the same contract on two venues, confirming the terms match, reading both at the same moment and checking both books takes longer than either price is stable for. By the time the second is found, the first has moved.

A note on what this is not

A gap between two venues is not a guaranteed return, and reading it as one is the error the guide spends its longest section correcting. Two populations disagreeing about a probability does not mean one of them is offering free money. It means the question is harder than either price alone suggests, and the reader who understands why is better informed than the one who does not. That is the whole of the claim.