"why Facebook chose to share the wrong metrics"

Why did you choose to include that bug into your code?

Srsly, people don't choose to make mistakes (but sometimes they choose to do cover up).

Also, there is no such things as "Facebook". Facebook didn't choose, someone inside Facebook either make mistake, or deliberate decision. "Facebook" is not single entity with single agenda and plan, but complex company, with different people optimizing for different things (i.e. security/privacy and growth and ads). There is shared high-level vision, but not every decision is made at single place.