The graph on the first page shows that a single MkII reactor has an efficiency of 2666.67, while an individual MkI reactor has an efficiency of 2500.
The first graph on the third page shows overall planetary efficiency. A MkII and a MkI reactor produce a total of 45000 energy, at a resource cost of -17/s. This corresponds to an efficiency of 2,647.06, a fairly minor drop.
Similarly, 1 MkII and 2 MkIs produce 49000 energy, at a cost of -19/s, corresponding to an efficiency of 2578.95.
Basically, it drops off slowly because the overall change in efficiency is very low, because the MkII produces so much more energy than the MkIs.
EDIT: Here's the data;
Mk Energy Cumulative Energy Resource Cost Cumulative Resource Cost Overall Efficiency
1 2 40000 40000 15 15 2666.66666666667
2 1 5000 45000 2 17 2647.05882352941
3 1 4000 49000 2 19 2578.94736842105
4 3 80000 129000 40 59 2186.4406779661
5 1 3200 132200 2 61 2167.2131147541
6 2 24000 156200 15 76 2055.26315789474
7 1 2560 158760 2 78 2035.38461538462
8 1 2048 160808 2 80 2010.1
9 2 14400 175208 15 95 1844.29473684211
10 1 1638 176846 2 97 1823.15463917526