From Diablo Wiki
Explanation regarding spawning probabilty
The information of the spawning rate is provided on the basis of 50 runs which yielded an exact 25/25 split. Naturally this is no guarantee for an exact .5 spawning probability, but the assumption of a .5 probability seems reasonably adequate anyway.
Edit: Another 50 runs, again 25/25. OdinFK 15:43, 18 June 2012 (CEST)