That's just what I heard from some people. Someone also said that the owner even admits that a some may be overvalued slightly (not sure if that's true) because she based trades on retired to retired item trades more than customs which means the value can be a bit higher than normal. She also states something like that in the disclaimer on her page that she based it on retired:retired and not customs. But I do agree that hers is closer to the real price for most of the items. I just wished she adjust the items more when they come out of season but that's pretty difficult to do when everyone is following the guides as law.
I always end up making a value board or fair trade board though since people can get a better idea of the current value.