I wouldn't trust the online tests either - too many variables to distort the figure.
I agree that there are several variables - however, the reported figures actually agree sufficiently closely to give reasonable confidence of the actual line speed at that time.
Namesco - 2.6Dn, 358Up
Speedtest.net - 3.01Dn, .35Up
Think Bb - 3.1Dn, 340Up
Doing one test in isolation gives little confidence in its accuracy, but doing several and finding them broadly in agreement increases that confidence.
SD