Gearset helps you view the differences in the metadata returned by the metadata API from your source and target. So users frequently ask us why there are differences in the comparison when users do not expect any differences.
This commonly happens after a sandbox was refreshed from a production org. According to Salesforce:
Refreshing a sandbox updates the sandbox’s metadata from its source org. If the sandbox is a clone or if it uses a sandbox template, the refresh process updates the org’s data in addition to its metadata.
However this does not mean the sandbox metadata will be identical to production. In fact, Gearset's comparison results will show you the difference in the metadata as returned by your 2 orgs.
Gearset can tell you the difference in the metadata, but is not really able to explain why they are different, for that you need to look at your source and target environments and your refresh process to understand how the org got the metadata it has. Fortunately, Gearset's compare and deploy function can help you make the target more similar to your source by helping you move the difference so that they resemble each other.