Gotta keep in mind that ANSI FL1 only calls for max measurement between 30sec and 2 minutes. I don't doubt it achieved 500lms, just not sustained. Based on this standard, even if it only hit 500lms for a fraction of a second and if that's what they achieved from an average of three samples, then that's what they can quote.
I know how we love to bash Chinese lumens but keep in mind SureFire was one of the mfg that helped come up with the ANSI FL1 standard and they don't measure their lights any differently.
As per my Runtime chart, I achieved 493 "lms" measured at 30 seconds on my PVC LMD (however it was still dropping after 30 seconds). It's no integrating sphere of course but I've been pretty happy with the results especially in comparison to other measurements I've seen for this contraption.
Again keep in mind that mfg specs are typically produced under the best conditions (just like the original EPA testing for automobile mpg before it was revised). They also don't provide the ambient temperature or age of the battery (for rechargeable's) which can have a huge impact on this type of testing.