.sxp file question?

I used some off-chip transformer for matching purpose in my LNA design.
I used the .sxp file provided by those suppliers. However, in my design the matching is not as perfect as when the measure the s parameter and generate the .sxp file

e.g they require 50ohm input impedance, while what I provide is 40ohm

How significant this would influence the difference between my simulation and measurement?