Explains why previous records still exist in the Activity table after you marked the Clear Stranded Activity Records check box on the Timeout tab.
After the Inactivity Timeout process runs, it will clear anything that has a date before the system date minus 2 hours minus the length of the timeout limit. For example, if your timeout limit is 60, then it will clear any ACTIVITY records that are at least 3 hours old. So, records from a few days ago should get cleared out.
If those records are not being cleared out, then it is possible that the Inactivity Timeout process never ran.
A user must be logged in to GP in order for the process to start in the first place. If you logged out of GP a few days ago, came back and looked in the Activity table a few days later, but you never logged into GP and stayed in long enough for the process to run, then the records will still be there.