On my Venu2 and on a user Forerunner 955 Solar, once 10K steps are reached, the Steps complication value is no longer a Number but a Float with the steps count divided by 1000, so 15500 steps becomes 15.5 steps. Not sure if it's a feature or a bug, but the documentation is definitively wrong if it's a "feature".
On that subject, in the Simulator, you can't test what happens when 10K steps are reached as the Complication value becomes a number in the billions. My uneducated quess is it also becomes a float but the bytes forming that float are read as a number, hence producing that result.