Seems a little strange. I thought there was only one class code for each flight. Wiki says:
Economy class codes
Full fare: Y, B
Standard fare: M, H, N
Special or discount fares: G, K, L, O, Q, S, T, U, V, W, X
On most airlines, unrestricted economy ticket is booked as a Y fare. Full fare tickets with restrictions on travel dates, refunds, or advance reservations are commonly classed as B, H, or M, although some airlines may use S, W, or others. Heavily discounted fares, commonly O, T, Q or W, will not permit cabin upgrades, refunds, or reservation changes, may restrict frequent flyer program eligibility, and/or impose other restrictions. Other fare codes such as X are restricted for use by consolidators, group charters, or travel industry professionals. However on some airlines W or X is used for frequent flier program award redemptions.
So I would have expected Y OR B OR M, but not a combination.
The Big Airways website site allows you to check the fare code when booking, and that only gives a single code for each leg.
I confess it has always been something of a mystery to me too.