I have investigated your issue further and may have found a bug within how the Test Case Transaction log shows the times of test steps versus the Project level transaction log that shows the times of the test cases.
I didn't find anything to indicate that the parallel tests aren't executing correctly just that the transaction log is not recording properly.
I have created a report to our developers to engage with them on this issue. I will update this post when I find out more.
Would like to know if this issue has been dealt with. I asked because I think I am having the same problem. Running keyword tests as a standalone garners a shorter amount of time complared to and running a group or running the whole project.