[LoadUIWeb] Validation against variables

I'm attempting to diagnose a problem in a web application, starting by recreating the issue.  The problem appears to be that the user's session will jump to somebody else when the server is under load.  I have created a scenario to test the situation and have a list of userIDs that I will be logging in as.  I've run the scenario and see that all the userIDs are indeed being used through the logging on the server. I saw the ability to do validation and have set up data extraction to grab the userID that the server sends back in the response as pageUserID. When I go to the validation tab, I see a dropdown for the value field and it shows the variables known to the scenario. If I try to use the userID variable, it puts @userID in the value field. When I run the scenario, I get the error  "...Validation failed.  The "pageUserID" variable does not equal the expected value "@userID".



Everywhere in the documentation it says you have to validate against constants, not variables.  If that is the case, then why have the dropdown and lead people to believe you can validate against variables?