Ok, I think I might have worked out why this anomaly occurs.
This situation would occur if the view variable window does not interact with the Betfair API, but the trigger logs record what result is returned from the API.
So in the instance I stumbled across, the variable window correctly shows the result of FIND ("Tennis","market_parent") as 1, yet if Betfair have misspelt the market name (as Mcbee showed earlier in the thread that they are prone to doing) then the trigger logs will correctly show that the result of that query is False.
Just posting this up in case it helps anyone who stumbles across the same problem, and I suppose the lesson is try and avoid using market name info in your triggers as Betfair have a habit of misspelling things.