That's why I said 'if' they are using the same system...
Basically I was told that if you have leave then the system will try to assign max hours so unlikely to get blocks of days off UNLESS you also bid ULRs that require said days off.
Of course rosters are much tighter for the CC but I'd guess the principle is the same. If you have two weeks leave then of course they will try squeeze as many hours as they can out of you. That generally doesn't provide for 5 days off before/after your leave...
What they told me (make of it what you will) that is when you have months with ground duty AND get what you bid for it is probably because the person manually assigning the roster has checked in the system to see what was bid. Majority of them don't bother which is why the roster almost always nowhere resembles the bids. All CC rosters with ground duty are manually assigned.
Again this was in relation to the CC rosters however I don't see why they would divert too much from this formula if it's working for *them*
It would also explain the issue of rosters being ****e whenever there's ground duty. Again as I said only speculation based on what they are doing with the CC but it makes sense to me