Don't forget they are 'spare buses' for this route so it is not uncommon to see them used. I do grant you they do seem to be used basically every day which makes the whole spare bus thing a bit silly (They may as well be put into Salt Road livery acting as spares for only the 144/A as that's all they seem to be put on!
Looking at bus times there were 3 Salt Roads that were not on the road today so that will be a contributing factor as to why 64033/5 were required. Backing up the fact that the spares should be in Salt Road livery is that whenever I check the tracker it is very rare to see that all 12 Salt Road's are out on-road or indeed on 144/144A's.