There was already the basis that he jumped from an E-rank. It's a logical judgement given that they know he's already changed once.GoblinGuy
Kind of a leap in logic purely for an author ass-pull. The guild-lion should have assumed he was hiding his strength the other time, and is simply showing more of his real strength now, rather than jumping to the (officially speaking) impossible, although correct, conclusion that some hunters can grow in strength.