Its because they started with "roughly 1km/s", which was deemed too difficult to understand for pleb readers, so it was converted to km/h, which is more relatable. Then it was converted to mph for the US audience, and the author just did a straight conversion without really considering the madness that is " about 2237 mph". I hate it when they do this.

1-2km/s, which is a reasonably accurate estimate for these things, should have been translated to "2000-4000mph" and it would have been perfectly good enough.