differences

I went right to our Walmart optical after having my IOP's read by my opthamoligist using a Goldman tonometer. The MD readings were 24 left and 23 right. Walmart used an air-puff and produced 28 left and 33 right. When I informed the Walmart optician of my experiment and the results, they remeasured the right, this time yielding 29. I know the relative accuracy and repeatibilty of these two instruments have been discussed before on this forum, but I feel these differences seem too large.