Have I encountered a bug in the tool or am I missing something in the rules?

I am working up a DONE4 ex-AMM for next year. The tool is completely satisfied with my itinerary and 'Book Now' yields a fare of US$7211 (+ 1243 tax), which is consistent with the JOD 5100 price quoted in various sources.

(I haven't actually tried to enter passenger and credit card info this time because I have met that bug in the past and expect to book this through a Canadian TA.)

Now, I understand the 'you must start and end in the same country' rule, but I also see that one of the exceptions to this rule is when both start and end are 'within the Middle East'. However, when I change only the end-point of my trip from AMM to MCT, something funny happens. The tool is still happy with the routing but 'Book Now' reports a fare of US$8463 (+ 1456 tax). I can understand that the taxes might be different (i.e., more); OK. But in this case it looks like the base fare being quoted is now the ex-Oman price, rather than the ex-Jordan price. But the trip still starts in Jordan!

So ... back to the question with which I began. Any insights appreciated.

cheers,

Henry