When deleting old entries, either manually, or via the auto-trash/delete functionality under personal data, the conversion metric gets corrupted.
The Conversion metric seems to calculate on Views/Entries, and the Entries number goes down. This doesn’t seem to be correct. A different number than current Entries should be used. This seems like a bug to me.
You can suggest to our product team any improvement you wish using the (+) button at the bottom of our roadmap page: Gravity Forms Roadmap - Gravity Forms
That way your request will reach the product team directly and they will be able to consider your request and queue it in the workflow of a future version if it’s accepted.