Who knows...but I have to assume the system in charge of aircraft positioning is relatively sophisticated. So maybe a 77A goes tech yesterday, whatever issue it is won't be repaired for a few more days, and CX's system knows how to reposition aircraft so they're placed optimally for their various missions. Presumably there will need to be adjustments down the line as the ripples are felt, such as your 77D sub on the SFO route, as that extra original plane is offline. Frankly, it doesn't even need to be a 77A that goes tech (I have no idea how it works in reality, but theoretically), instead it could be that a 74A or another aircraft/issue but whatever it is it has consequences throughout the CX aircraft inventory, ultimately forcing a 77D swap on SFO.