I'm just working on some welding projects in between the snow here so I will post a video later today, but here's what I've observed

Power the printer and connect to repetier. At this point the head is in a random position on the bed.
I type in a single g28 and it goes to 0,0 and homes thru read at 0,0 and then retracts the probe and stops. It does not return to Centre of the bed

Any additional g28 will then return to the Centre of the bed.

What I observed last night was that after I connected, if I used repetiers controls to home the x and y axis first and then run my initial g28, it would then return to the Centre.

I assumed that a g28 command was the same as homing each of thru axis manually, when in reality I still needed to home the x and y axis before issuing a g28...if that makes sense!