The value typed by the user is not the value received by the completer.
Imagine a FilePath completer that deals with directory files.

cmd --opt1=/my\ path/myfile.<TAB>

The completer receives "/my path/myfile.", there is now a difference of 1 character, the length of the value to complete is meaningless and doesn't allow for offset computation.
There is no way to compute an offset in order to complete after the "." or after the "/"