Volume counts value traded. Nothing about the measurement asks whether the two sides were the same person, and on-chain that is straightforward to arrange: buy from yourself, sell to yourself, repeat.
The cost is fees and gas. The output is a volume figure of whatever size you are willing to pay for.
Why depth is different
To make a pool look deep you must actually place capital in it and leave it there — exposed to impermanent loss and to anyone trading against you. That is expensive, continuous, and genuinely at risk.
So between the two, the one that is costly to fake deserves more trust. That is the practical core of pool depth is not volume, stated as a question of incentives rather than definitions.
Why anyone bothers
Because rankings are volume-sorted. Listings, aggregator front pages and trending lists frequently rank by turnover, so manufacturing turnover buys visibility. The incentive is structural rather than exotic.
What it looks like
Volume enormously out of proportion to depth is the first tell — though that is also the ordinary state of a genuine discovery-stage token, so it is a prompt to look rather than a verdict.
Better tells are structural: turnover concentrated in very few addresses, or a suspiciously even flow that does not vary with the market around it. Real trading is lumpy and correlates with everything else; manufactured trading often does not.
The rule
Size on depth, use volume only to ask whether anyone is there, and treat any ranking sorted purely by turnover as measuring willingness to pay fees. That is the same caution as excluding plumbing before ranking — a leaderboard is only as good as the thing it sorts on.