Question concerning Blue Line cars

Tonight while browsing nycsubway.org, I noticed something about the Blue Line pantographs. There seem to be two different styles of them, one being a single arm, and the other is the more traditional type that I've seen on the Green Line, and on most other electric railcars, LRT's included. The only other place I've seen the single arms is in Europe or Asia. What is the reason for this, or does anyone really know? I'll link some pics for examples.

One with the standard type pantogragh



One with the single arm.

I asked this question to Ted McAuley, a Superintendent at Orient Heights Carhouse who was an official guest of this forum a few weeks ago (he's also a regular member).

Here was his answer:

http://www.railroad.net/forums/viewtopi ... 2&start=14