is this accurate

When I compare the basal metabolic rate on this site to an external one there is like a 400 calorie difference. Does anyone know why?

Try This:

Katch-McArdle:Considered the most accurate formula for those who are relatively lean. Use ONLY if you have a good estimate of your bodyfat %. BMR = 370 + (21.6 x LBM)Where LBM = [total weight (kg) x (100 - bodyfat %)]/100

or this:

Mifflin-St Jeor: Developed in the 1990s and more realistic in todays settings. It still doesn't take into consideration the differences as a consequence of high BF%. Thus, once again, it OVERESTIMATES NEEDS, ESPECIALLY IN THE OVERWEIGHT.
MEN: BMR = [9.99 x weight (kg)] + [6.25 x height (cm)] - [4.92 x age (years)] + 5
WOMEN: BMR = [9.99 x weight (kg)] + [6.25 x height (cm)] - [4.92 x age (years)] -161