Tie Break points not making sense

I thought I understood the tie break points but the help section has confused me.

The example they use I have replicated below. Also have included the link for verification. http://www.chess.com/tournaments/help#tiebreak

Bob and John points work out as I suspected however, Mary's - I don't get it. Could someone please explain Mary's Tie Break points to me??

Group#      1         2         3         Points   Tie Break
1. Bob       X        0 1      1  1          3         4.5   

2. Mary     0 1       X       0  0.5      1.5        3.75

3. John     0 0      0.5 1     X          1.5        2.25

Thanks.

Mary: (1/2 Vs 1 = 3) + (0.5/2 Vs 3 = 0.75) = 3.75

Thanks Doggy_Style

I was having one of my 'vague' moments. It now makes sense.