The erroneous transformation

Sarillia was not at all supposed to transform into a Forest-Jay, the day she turned seven.
She should have turned into a Town-Jay, just like all the rest of her family had done it, as long as anyone could remember.
No one knew how this could happen ... Was it perhaps because she had uttered her transformative rhymes quite flawlessly, except for a single letter?
Or was it because The Unmentionable had chosen Sarillia to be a part of their deadly game in which only they knew the rules?