102(b) quesition

Is anyone here can tell me why an earlier \"foreign convention date\" cannot be used as an Effective Filing Date to determine whether there is a statutory bar under Section 102(b) ?

do you refer to this ?

§102. Conditions for patentability; novelty and loss of right to patent
A person shall be entitled to a patent unless--
(b)
the invention was patented or described in a printed publication in this or a foreign country or in public use or on sale in this country, more than one year prior to the date of the application for patent in the United States, or