In laymans terms I think you are spot on. I've also noticed similar movements close to departure, but more obvious ones, such as Y sold out but seats further ahead (F6 J4 W0 Y0 lets say) and then in the morning suddenly its (F6 J2 W0 Y2) where the system has obviously 'rolled' the availbility based on the likelihood of selling more Y tickets at that stage exceeding that of selling more J.
One could do a DPhil on understanding it all!