A quote is indicative, not a locked price
A quote reflects conditions at the instant it is fetched. It reads the current price of the pair, the liquidity available across venues, and the estimated gas, then computes what you would receive. All of those inputs change moment to moment, so refetching naturally produces a slightly different number.
This is the same reason two providers can quote differently at the same time. Everyone is pricing a market that keeps moving. A changing quote means the market moved, not that the tool is unreliable. Treat the number you see as an accurate estimate for right now rather than a promise for later.
What moves a quote
A handful of live factors drive the changes you see between one fetch and the next.
- Price movement in the underlying pair as the wider market trades.
- Liquidity shifting as others trade the same pools, which changes depth and price impact.
- Gas estimates rising or falling with network demand, which affects your net received.
- Route changes when a different path becomes the better one to take.
- Order size, since a larger amount takes more of the available depth and moves the price against you.
- Quote expiry, since a stale quote is replaced by a fresh one that reflects current conditions.
How slippage tolerance absorbs the drift
Because quotes move, a swap carries a slippage tolerance. This is a limit on how far the execution price may drift from the quote before the swap will not go through. Within that band, small changes between quote and fill are accepted. Beyond it, the swap reverts and your funds stay put, which protects you from a much worse fill than you were shown.
A tolerance that is too tight can cause repeated reverts on a volatile or thin pair. A tolerance that is too loose leaves room for a worse fill. The right setting depends on how much the pair moves. Stable pairs need very little room, while volatile or low-liquidity pairs need more.
How to act on a changing quote
A changing quote is a market to manage, not a bug to fix. Act on a fresh quote rather than one that has been sitting on screen, since a stale number is the most common surprise at execution.
For volatile pairs, expect more drift and set a sensible tolerance so normal movement does not trip a revert. If a quote looks far off from what you expected, refetch and compare before committing. Swapping in calmer conditions, on a fresh quote, with a tolerance matched to the pair, is how you handle drift without overpaying or getting stuck in reverts.