"it must be because we did the testing when it was cold and high pressure so the air must have been descending," comes from very clever people who I respect, yet sounds odd to me.
That suggests to me that what is being criticised is primarily your analysis, or more particularly how accurately you have reduced the test data to "standard conditions" - or in other words the validity of your comparison to previous test data.
How hot, cold, high-pressure, low-pressure it was then is only relevant insofar as you should have done enough measurement to establish actual test conditions throughout your own testing - it's the analysis that then becomes important. (And obviously that you tested the right things at the right conditions - for example are you absolutely sure there was no external change that might have affected the static, and thus the indicated best climb speed?)
G