Not sure if this is a bug, but, for the things you buy that automatically buy one of the lower level thing, I expected that it would buy one for each of the thing you have. e.g. if I have 50 Rivers, it would buy 50 Streams every time the bar fills up. But it doesn't... perhaps the number that it auto buys is equal to the number of Rivers that you've MANUALLY bought, not the actual number? Here's a screenshot that hopefully makes this evident: 
Thank you for the comment and especially for looking 'beyond the surface'.
It's not a bug. :-)
As with the PP, there is also a logarithmic formula behind it. That means for the first generators you get the underlying item almost 1:1 and to increase the growth rate you need more and more generators.
The formula behind it: (int)Math.Ceiling(1 + Math.Log(gd.Level[i],2));
I'm looking forward to your next post. And if you have time, I'd like you to be a beta tester for my next game. :-)