Quote Originally Posted by Ken White View Post
It's one inch at a thousand inches, one foot at a thousand feet, one yard at a thousand yards. Better for all purposes than rather sloppy degrees. It's handy...
And one cm in 1000 cm, one dm in 1000 dm etc. (just another stab at US reluctence in dropping that silly imperial system)

Seriously now, as your link indicates, it is a rounding off. If pie is pie and a metre is a metre, than 2 pie radians equals 6283 mil. So stepping out 1000 and one sideways will not give you a full circle if you do it 6400 times.
Accurate enough for army but I assume that surveyors, navy and air force avoid it perhaps for that reason.