That does not necessarily mean that they are inaccurate, however. Once you know that your 1/500 is really 1/160 or whatever, then just use that. If you really need reliable speeds faster than about 1/125, get a camera with a focal-plane shutter.
FWIW I test my shutters by sound (ms between opening and closing peaks) and have very consistent results through the range of speeds. I use mostly smaller apertures, and thereby eliminate much of the opening-closing light loss. When I get my shutters CLAd, I always ask the tech to check the shutter speeds optically at f/22 and give me a list. This goes on the lensboard so I can use it in the field.