The variable is declared in the "Main" group in my example (see the .capx). So this group should be the scope of the variable, not the on start event. Quote from the manual:
"For example, if an event variable is in a group of events, it becomes a local variable. Then, it will only appear as an option for a variable in events inside that group. In other groups or in other event sheets it does not appear at all and cannot be accessed. This makes the variable local to the scope in which it is placed."
Also from the manual:
"By default, local variables reset to their initial value whenever entering their scope (usually every tick), like local variables in programming languages"
So the variable should be reset only when entering the group.