Value of variable set by custom script not available to other columns?

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I've searched around a bit, but I haven't seen an answer to this one...

I have a custom script that is setting a variable in the process tab. I have a column that displays that variable, so I can see it's getting set correctly. Then I have another column that uses that variable in an IF statement to decide what to do. It seems that the variable is not set at the time when the IF statement is evaluated. To verify this, I added a simple test column with the following definition:

if ($TotalIVRSessionCount > 1, 'multiple sessions', 'one session') 

In the attached image, you'll see I always get "one session", even though there are clearly more. Am I missing something?