It's the Q fare on outbound that is mostly driving higher price. It's not a married segment inventory issue as there is only Q bucket open on standalone LAX-BOS flight too (as seen below). Comfort+ fares are dual inventory fares. You need to look at both W/S C+ buckets and the coach buckets to figure out which particular C+ fare is bookable as they have both W/S booking class and underlying coach basis class (first letter of fare basis code). With only W and Q coach class open, only C+ fares with W booking class and Q fare basis class would be bookable.