I've spent a lot of time lately watching fare buckets, as I started keeping an eye on R/RN for an upgrade on an upcoming flight. But while watching them, I've noted a lot of other strange fare bucket mechanics occurring, and am wondering if there's any rational explanations for them, other than: a) airline revenue mgmt whim, or b) IT systems errors.

First, there's this thread I started a few days ago, when I started noting discrepancies:
Fare bucket inventory discrepancies CO vs. UA vs. booked?

(Note to sbm12 -- it turns out that the view on the existing reservation in "View Current Reservations" is the most accurate of all, even though it omits buckets with inventory of 0. At least as far as R/RN is concerned, I snagged an upgrade with R1 showing on my current itin, where searching for the flight showed R0 in 100% of the cases I tried.)

Second, I'm noticing that the lowest-available fare bucket isn't always made available for sale on a single ticket, even if bookable separately. Sometimes the CO.com website will combine buckets on Int'l trips for you, and sometimes it won't. UA.com vs CO.com give different results, in this regard as well. You can also book the segments at the lower-priced buckets as individual tickets, while sometimes those prices are even higher.

For instance: CLE-ORD-HKG on 1/12, returning 1/18. Search CO by price and you get fare buckets K-K outbound, V-S return, at a total price of $1406. Build by segment, and you get Q-S/V-S at $1607 as the lowest available fares -- and if you click on the Fare Class display link, it will show you "K0" for the first two flights, EVEN THOUGH searching by a different method or Co.com or on UA.com will give you the lower fare and show availability K9 in those buckets for those segments.

The thing that really kills me here is that this happens with the R/RN buckets as well, not just revenue-purchase pricing. Searching certain city pairs will show different R-bucket results for the Int'l segment. Once you've booked it, the results under "View Current Reservation" seem to then be consistent. But there doesn't appear to be a reliable way to know in advance of purchase. I've let my ExpertFlyer account expire, but prior to it dying, EF seemed to be agreeing with the Co.com _search_ results -- which, as just noted, do not seem to be accurate in all cases.

Anyone else have experience with this? I understand getting different pricing from different sites and search methods. But any idea why fare buckets, including upgrade fare buckets R/RN, would be showing up differently for the same flights, same carrier??