For all the speculation here, it seems to me that the algorithm and the practice have changed a bit over time. Nobody here can predict what it might be tomorrow and what exactly the triggers are. One possible explanation of variance is that the algorithm looks at the loads and does not act until the seats are likely to become needed. If that is the case, the opinions here are even more speculative because nobody knows what the load on a given flight will be a week from now.
The solution if one has to purchase tickets which may push the envelope a bit is to call in and assure that WN is satisfied and has made PNR notes to the effect that the tickets are not speculative.