First, questions like this must be posted here: www.elitefourum.com/t/the-giant-professional-grading-thread/30930/1
Second, as has been pointed out, prescreen grades are estimates. For a card to be one grade off of the Ludkins estimate is quite common as professional grading is subjective. It is well-known that PSA will (very rarely) damage cards and their compensation process for it is very bad. I certainly don’t think Ludkins is responsible for compensating you. A refund of the grading fee is more than I would expect.