Why are the theoretical and measured frequency responses off a low pass filter different?
What is the reasoning behind this other than measurement errors?



tolerances of components
wrong theoretical calculation
incorrect measurement procedures (wrong impedance scope-probe)
test equipment errors (out of tolerance or calibration)
human errors (mis-reading numbers / figures)
implementation error (such as impedance of signal generator)
implementation error (incorrectly wiring the filter)
implementation error (someone using the wrong components)
parasitic components
skin effect
loop inductance effects
mis-reporting of results
EM interference
didn't take account of transmission line effects
someone lying
someone a bit deaf or can't read the numbers written down
espionage (deliberate doctoring of the numbers or the test method)
an extreme case of Heisenberg's uncertainty principle
red shift
drugs were involved
it was all a dream
theoretical vs measured were slightly different but within sensible expectations