Yeah... after seeing another source that seems to be what the scientific community agrees on. I too think it has something to do with cordinate covalent bonds. Check:
http://www.chem.purdue.edu/gchelp/gloss/ccbond.html
Try this, draw 3 O's with 7 dots and one O with 6 dots. The phosphorus in the middle has 5 dots you can draw to make it a total of 32. For the three O's with 7 dots, phosphorus only lines up 1 dot for each. 2 dots are left which it shares with the O with 6 dots. I guess that counts as a double bond.