I'm using stb-tester to measure the latency of my set-top box UI. The test script looks like:

def test_measure_latency():
    stbt.press("KEY_MENU")
    start = time.time()
    m = wait_for_match("menu.png")
    print "Latency is %ims" % int(m.time - start)

I am consistently seeing the stb-tester timing of latencies that is 50~70 msec less (i.e., faster) than the value when analyzing a IR detector / high-speed (240 fps) video camera capture of a connected TV.

What is the reason for this?

^{Disclaimer: I work on stb-tester and am an employee of stb-tester.com Ltd. This is a sanitised version of a question we've previously received through other support channels published here in an effort to seed a stb-tester support community on stackexchange.}