In which case one or more of the following is likely to contain an error:
(1) The test set
(2) The static
(3) The pitot
(4) The relationship between actual OAT and your estimate / measurement of OAT in determining density altitude.
(5) Your maths
(6) The accuracy of your airspeed maintenance / measurement.
To be frank, given that lot, I'd consider myself pretty fortunate to be within 5 knots of where I should be on a fairly quick and dirty single point flight test.
I probably sounds like I'm shrugging my shoulders. Plead guilty as charged m'lord.
G