Since the outlet is labeled 3 phase; what you may be looking at is a 240 volt, 3 phase, Delta, 4 wire (center tap of one winding is neutral). I just ran into this a couple weeks ago. I agree; this is wrong receptacle for this application. What you may have is:
Assuming G is neutral:
1. X to W = 240V
2. W to Y = 240V
3. X to Y = 240V
4. W to G = 120V
5. Then either X or Y to G = 120V
6. Then the other one in #5 above to G = 208V (wild leg)
Look at this website for more
http://en.wikipedia.org/wiki/Split_phase
