It is a puzzlement, but honestly not a surprising outcome if you're using the online tool. A couple of things:

Somehow the fare details you appended are for a Global Explorer trip, not a Oneworld Explorer. This is Oneworld's mileage-based RTW product (maximum 34,000 flown miles in business class) which has a base price a few dollars/pounds cheaper ex-Norway than the Oneworld Explorer, which is maybe some decision made by the Oneworld robots based on the mileage of your trip. The Global Explorer is far less flexible than the Oneworld Explorer; if you were to amend your plan to include more stops and more mileage, you'd have to convert the GlobEx to a Oneworld Explorer (OWE) with, of course, some major service charges.

Regarding the big number for carrier surcharges, not having a breakout by segment is a major problem, but I took it upon myself to use ITA and look up the YQ and YR charges listed for the various segments and carriers on your itinerary. These are shown in the currency of the departing airport, so one needs to change them to Norwegian Kroner to see how the total compares to the NOK 28,002 total on the estimate. And, guess what... the numbers don't add up at all. The total YQ/YR surcharges using he segment-by-segment approach comes to NOK 10,842, of which NOK 8934 is from one segment - BOS-LHR (US$850, the standard YQ charged by all the Oneworld carriers on USA-UK business class segments, whether it's LAX-LHR or BOS-LHR, LAX-MAD, JFK-HEL, you name it.) By comparison, the YQ/YR charged by Oneworld carriers on Canada-Europe business class segments is CA$714.50, or US$536. From the Caribbean it's US$365, from Mexico it's US$400.

My hunch - and this is just a hunch - is that when you put Finnair in as the first carrier, somehow the fare calculation and ticketing duties get assigned by the robots to BA. (This is similar to the Star Alliance online RTW tool, which assigns everything to Lufthansa for ticketing, whether or not LH even has a segment in the trip.) BA is notorious for adding YQ surcharges on segments flown by everyone in the itinerary, even if those carriers don't charge YQ or YR themselves. (Note I don't know the difference between YQ and YR, but they both turn up as "carrier surcharges," so I just lump them together.) Anybody who's used BA Avios for award travel knows that even on non-BA segments, BA whacks you with big surcharges (incorrectly called "taxes" by many) as a form of revenue enhancement, i.e., profit-seeking, on BA's part. My guess is that that's what's happening here.

Possible workarounds, no guarantees or promises offered: Redo your itinerary so that it will be palatable for American to do the fare construction and ticketing. First, make sure it's a Oneworld Explorer you're buying, not a Global Explorer. Second, put in an AA-operated over-water segment, not an AA codeshare on BA or QF metal. SYD-LAX on AA 72 would get my vote but it's your call. You might also add a BOS-YUL (non-Oneworld so outside the ticket) segment then fly YUL-LHR to get the cheaper ex-Canada transatlantic YQ charge. You can afford the surface segment. (Or you could add two segments in North America and fly BOS-ORD-YYZ, or if AA is flying its seasonal route to Cancun, go BOS-CUN-LGW)

There are various workarounds, but the main thing would be to get someone besides BA to do the ticketing. Like I say, no guarantees but maybe worth some time.

I'm attaching a pdf of my worksheet. Hope there aren't any gross errors.

Edited to add - I see I left out the legitimate taxes and non-YQ/YR fees from the fare quote, so the bottom "all in" line on the spreadsheet is bogus. But the math is easy.