I setup a hom callback on a string field.

when i enter a value the callback is invoked.
when i delete the field, and hit enter the callback is invoked.
when i delete the field, and change focus the callback is invoked.

* when i RMB on the parameter (with data) and revert to defaults
(the empty string), the callback is NOT invoked.

has anyone else noticed this?
is it a bug or is there a reason for this behavior?