Google, when searching for
executable define or
executor define shows that second syllable is stressed, i.e. exEcutable, exEcutor.
Speech on http://www.thefreedictionary.com/executable suggests that
exEcutable is British and
execUtable is American.
Single variant on https://en.oxforddictionaries.com/definition/executable is
Executable as British and
execUtable as American.
All my colleagues --
Is it dependent on whether it's British or American?
Can it be generalised to more common rule?