This is the case where fans depict the characters having limitless potential with their abilities. Things like, Cirno freezing below absolute zero, Reimu flying out of reality, Yukari with her limitless border manipulation, etc. Some are actually canon, some not(maybe). For Byakuren, she just happens to have the ability that increases her physical abilities. This usually results in fans giving her unlimited buff. She can have limitless strength(stronger than Suika/Yuugi/You name it), durability(tougher than Tenshi/Oni), and speed(faster than Aya).
Some of them are actually pretty nicely explained though in some doujin I've read. For example, in a long race Aya will still win, but in a sprinting contest she'll lose because Byakuren has insane acceleration speed. Compare Peerless Wind God and the Superhuman spellcard and you'll see what I mean.
