I'm being a bit paranoid here and might be wrong, but what if it is calculated by multiply the percentage of your mark by the weighting and add them up?
E.g.
1.80/10
18%
18x12.5%=2.25
10.00/20
50%
50x12.5%=6.25
7.50/20
37.5%
37.5x12.5%=4.6875
7.50/10
75%
75x12.5%=9.375...