North American fares are exclusively priced "per direction". There are no restrictions on tickets such as Saturday night stays, round trip requirements, etc. The overseas fares are still priced based on round-trip fare rules and are not constructed "per direction".

Without knowing all of the fare basises that you were bringing up, its hard to give a definitive answer to your question. However, the booking engine, for example, might be constructing that cheaper J fare as a one-way add-on to the PY fare, ie: two separate fare basis', while the pure roundtrip fare might be using a roundtrip fare basis or might be constructing the fare with an allowed open jaw.

Its certainly possible that if you dealt with a properly motivated travel agent who could look up the rules and optimize accordingly that they'd be able to do optimizations that would not explicitly come up in the "consumer-friendly" online reservations tools.

You are probably up against a fare validity rule as well, because of change of season, expiration of a promotional fare, or simply fare bucket availability, when it comes to the wide discrepancy in pricing between the outbound and the inbound. $1308 YYC-MUC in J is a fairly deeply discounted J/P ticket, a price with limited availability, a lot of restrictions, nonrefundable, etc. $5k each way (ie: $10k) would be a full-J on that route, fully refundable, changeable, books into J, etc.