Now I'll explain you:
Each 2 weeks cost 100 diamonds. Infact 1 month costs 200 diamonds. So each week costs 50 diamonds.
Now 4 months cost 600 diamonds, namely, 16 weeks cost the price of 600/50=12 weeks.
8 months cost 1000 diamonds, namely 32 weeks cost the price of 1000/50=20 weeks.
So calculations aren't wrong.
I hope I've explained in a simple way.