I got the answer in SRV.9.9.2 !
If no error-page declaration containing an exception-type fits using the
class-heirarchy match, and the exception thrown is a ServletException or
subclass thereof, the container extracts the wrapped exception, as defined by the
ServletException.getRootCause method. A second pass is made over the error
page declarations, again attempting the match against the error page declarations,
but using the wrapped exception instead.