Wait are you pointing it to a white background? Also, 88 watts is in no way powerful enough to adequately heat a 2 foot high by 2 foot deep by 6 foot long wooden cage. Something went wrong in the calculations you gave to Pro Products and you need to contact them again.

White is horrible for absorbing heat Try putting a darker substrate even dark brown paper in there and try testing again. See if the reading off it is higher.