Scripts in variable dependencies and interval expressions are re-run whenever the running of the scripts causes a value to change. The reason for this is in case the new value would have caused a preceding part of the script to evaluate differently.
For example:
#myVar = #myVar + 1;
(where "myVar" is a variable's code)
Every running of the script will change the value of #myVar, which will cause the script to be run again.