It makes a lot of sense to me, the difference was most likely due to your nocks being too tight on the non-fastflight string.
The fastflight string might have made a difference for two reasons, one the nock did not fit as tight, due to the smaller diameter of the fastflight string.
Two, the increase energy may have nulified the effect of the tight nock with it's increase energy transfer.
Remember, it is wise to make sure our nocks are not too tight regardless the type of string we are shooting.
I am happy it worked out for you....