Mr T
The figures are determined by a test pilot but they are figures that an average pilot should achieve on a new aircraft. The reason for using a test pilot is that they will fly accurate profiles, correct speeds, correct reference point, shut the throttles when the manual says etc. and do it in a repeatable way. The performance figures are not the very best numbers that its possible for a pilot to achieve - you or I should be able to achieve them on an average day and if that wasn't true the manufacturers would get the crap sued out of them the first time someone went off the end because the data would be deemed unrealistic.