Thanks everyone...
I feel like an idiot..
But -- for the dates or fares I happened to be looking at, indeed the suggestion from
emcampbe was true....
"May very well be that that the cheapest price for the full round trip is W (or whatever the lowest upgrade-eligible fare class available is on the outbound). A K fare (for example) on the return may not be combinable with the W (or whatever) on the outbound, or the K fare available that will combine with a W May be higher than a W both ways."
anyway it was confusing me, I thought it was forcing the W fare into the table because I asked for it in the search. It also confused me because the search attributes aren't segment of the trip specific, so I thought it was just giving me a blanket W fare regardless.
Thanks all for the kind responses.
YTJK