That is an odd one, but it usually means actual "days in column", rather than "days in column in sprint".
Imagine a workflow like to-do -> ready for dev -> in progress -> ready to test -> done, and you have 5 columns mapped with the same names
If someone picks up an issue and shoves it into "ready for dev" without it being selected for a sprint, it could be weeks before it gets into a sprint. When it does, it will then show as being in the "ready for dev" column for weeks because, technically, it has been. It just hasn't been on the board because it wasn't in the sprint until now! The column is equivalent to the status and it has been in that status for weeks!
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