Can't figure this out, though in this case it looks like it would work out in my favor. I'm trying to learn more about how the fare rules work, can someone explain this one to me:

Looking at a trip with my girlfriend, NYC to Tel Aviv on Fri 8/17, to Zurich on 8/21, and back to NYC on Sun 8/26. Looking to go across the Atlantic in J, not concerned about fare class for the 4 hour hop. Looking at just the TATL segments, UA prices the multi city round trip at $3969/person (UA90 in Z to TLV, UA135 in P ZRH to EWR). For the TLV to ZRH flight, Swiss flys nonstop, LX253 is $772 on the Swiss website offering an Economy Flex ticket as the lowest class (not sure how to get fare class from the Swiss site), or offers $1546 in business.

On the UA website, the same one way LX253 also prices at $772 for economy (M fare showing M9) and $1546 for business (C fare, showing C9 but also D9, Z9, P9). So the total is $4741 for the two flights, business on the long legs, economy on the short one.

On the UA website, I'm not able to book a multi city itinerary with the middle flight in economy. After I select economy for the TLV - ZRH leg, I'm told Business is "Not Available" on the ZRH - NYC leg (despite that leg still showing J9 JN9 C9 D9 Z9).

When I build the multi city itinerary with all three flights, but select the business (P) fare for the TLV - ZRH leg, the entire trip prices as $4229. Or $512 less than it would have with an economy ticket for the TLV - ZRH leg.

So finally getting to my question, I was under the impression multi city flights priced as sequential one way tickets, why does buying a business class ticket end up saving me $500. Or is this an odd anomaly or bug I stumbled upon?