Aha! So there are other people who are OCD when it comes to knowing item distribution in games! I just did this two nights ago and came to see if someone had already.

Of course, the thing I'd be interested in finding out would be the rationale behind this particular distribution. I'd assume that it's mainly for realism (more jungle and grassland, less rock and lake), but that still doesn't explain why there are 6 jungle/grass pieces but only 5 grass jungle pieces. Similarly with Lake + Jungle and Rock + Desert.

Probably, it's an arbitrary decision because the designer wanted to have an odd number of pieces with those terrain combinations?

Notice that in the RGG rules, the 5 types are listed in the same order as their frequency under the rules for expanding a settlement.