[Fyron]

Moons have "any" for planet type, which will explain most of the discrepancy in the planet type totals.

I don't believe SE5 does anything to make sure each atmosphere or planet type ends up with a balanced distribution of sizes. It is supposed to keep each individual aspect distributed closely to the percents at the top of SystemTypes.txt (for planet entries with "Any" attributes), but nothing checks to see how they come out together. Individual maps can be quite skewed by size distribution, as you can see in your data.

Yes, it is true that there will initially be more breathable gas giants than other types due to the lack of gas/none. It is also true, however, that it is entirely possible to start in a position on the map that is devoid of breathable planets, or saturated with them. Reducing the number of gas giants actually gets fairly complex, due to how the data files are generated.