I (distantly) knew someone who was cutting grass on an incline adjacent to a pond. The lawnmower flipped, and it pinned him in the pond. He drowned in a foot of water and was sober.
People misjudge things all the time. I would guess it was a combination of distraction and/or non-ideal terrain (slope, dropoff, or combination).
The linked page uses the Mifflin-St Jeor Formula which appears to have been published in 1990. As someone acquainted with eugenics but not how BMR calculations play a role, could you share some I formation on how the two are related?