Slip and slip catching

Ponting on the recent slip catching woes:

 

"We've had guys moving all over the place in the cordon. Andrew Symonds coming in, Michael Clarke moving around first and second, me the other night being probably a specialist second-slipper finding myself at first slip and I put one down there."

 

I don't understand the fine distinction. I don't think there is much of a difference standing in the first slip and the second slip or for the matter any slip. How you position yourself for the catch is what matters.

 

Can slip catchers throw some light?