Both fares and fare class availabilty vary based on point-of-sale for international flights like this. When searching roundtrip ex-CAI, both outbound and return fares and class availability will be based on CAI point-of-sale. With two one-way bookings, only outbound CAI flights will use CAI based fares and class availability, while return flights will use ex-US fares and class availability. So this in not exactly news or unheard of. If we knew dates we could investigate further and check fares and availability on EF.

With domestic fares within US (and US to/from Canada), fare filings are symmetric and there is no difference in class availability depending on origin. Just with long-haul international, however, there can be cheaper fares which require roundtrip bookings (on both domestic US fares and fares between US/Canada). Also, booking roundtrip between US and Canada will use same currency in both directions based on country of origin, while booking as two one-ways will use US dollars in one direction and Canadian dollars in the other (the fare amounts in dollars are actually the same in the fare filings). There are also tax implications between booking as roundtrip vs. one-ways between US/Canada. But in the end, it's not unusual to see cheaper options when booking as two one-ways when flying ex-US to Canada vs. a roundtrip ex-US due to the stronger US dollar.