@muneer66 Oh! You’re right!
“set app [app VarCompare]” actually takes the value of the variable “app VarCompare” and uses it as the variable name in the “set app” block. For example, if “app VarCompare” had the value ‘true’, then “set app [app VarCompare]” is evaluated as "set app ‘true’ " where ‘true’ is the name of the variable.
Confusing, to say the least, but expected behavior now that I understand it better.