AFAIK the variable is processed when the player is loading the file and not up front when generating the playlist (because elements may change after the generating happened)…
Did you try to load a test playlist (with recent variables for the testing hour) and see if the playlist changed?
By the way…
There’s a new function started in 7.0 that might be a better solution for you instead of using variables: Events - use latest file in folder (scroll down to see more Info: 
You just need to add an automatic container to your database and include it as an element in your hour template instead of using an element with variables…
You’re welcome!