Please see attached files.
The same Module:
Different Module:
Why is the result of calling function from the same or different Module is not the same ?
If this is a joke, then this is a very funny joke 
Nobody forbids you to use such a good name for your function - IsNumeric(). But, unfortunately, this word has already been reserved before you - see here
If Basic knows which function you mean, it uses your function, otherwise it uses its own (the one that returns TRUE or FALSE).
Try to explicitly specify that you want to execute your function from the ModuleFunction module:
Dear @JohnSUN,
OMG! It’s the reserved word.
I will immediately change it to IsItNumeric.
Thank you so much, Teacher.