When you run a user validation the validity status of the replaying depends on the HTTP status response of the server.
In other words you can have a check user valid but it does not mean that the script is really working.
The difference column will compare the recorded response and response during the check vu of each request.
That column helps you to quickly identify where a script is not working. If you have a 100% difference this means that the server sent a completely different response compared to the recording. That could be the consequence of an issue.