I have similar, but reverse question. My step count is high in Health, but seems normal in the 3rd party app StepsApp. For instance, on November 25th, StepsApp recorded 8276 steps, which is about right. Health, OTOH, logged 14,759 steps. There’s no way in the world I had that many. The day before, Health logged 15,514 - no way - and on it goes.
I contacted StespApp support, and they said that they had had reports of high counts in Health. At pne point, my count was 40,000 steps high.
I don’t have an Apple Watch, so that can’t be the cause.
In case someone is keeping track, please add me to the list.