I'm using [stb-tester](http://stb-tester.com) 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?

<sup>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.</sup>