Yes, there are rules. You can find them in the Great Library at the Apolyton site. IIRC, the odds start out basically equal for tech/gold/barbs/nomad or AT/unit. Certain circumstances will then enhance or suppress certain results:
1. Before building a settlement, you will not get barbs. (Theoretically, you should not get AT, either, but in my D+ games, AT are a pre-settlement possibility.)
2. If you have no settlements on that continent, you are more likely to get an AT.
3. After Invention, you will no longer get tech (tech probability then gets added to gold).
4. After Explosives, you will no longer get nomads.
For more, check out 'Poly's Great Library.