Parson was previously required to give truthful answers to questions asked by Charlie. However, these answers were to the best of parson's knowlege. As Parson was unaware of these spells and other resources being disclosed, he was unable to enter them into his calculations; essentially letting him "lie" to Charlie without lying.


If Charlie bases his actions on his previous incorrect knowlege its highly possible Parson could bait him or Ansom into overextending themselves andt hen spring their hidden weapons on them.


As far as literary options go, this is a good way to allow for Parson to defeat the forces in front of them without invalidating information stated to be true earlier.