The first few episodes of QAF mirrored the British version of the series and then the North American writers started taking things in a different direction.
They decided that the series would be easier for them to write if Brian never told Justin that he loved him and they just pretended that quote never happened. It was beyond their comprehension that their fans would notice.
So the real reason that Justin claimed that Brian said "I love you" is that the writers were just winging it...changing time lines, birth dates, character personality traits without any sort of planning or consistency.