Some routes just go into hiding and don't want to be found again, to the extent that they'll have several changes of postcode. In 1922, the A648 was assigned the route between Brighouse and Denholme in West Yorkshire. But it had tired of this vocation before long, and by 1953 the A644 had been extended along this route, obliterating the A648. It reappeared in 1968 as the new link road between M1 junction 24 and Nottingham. There must have been another change of mind, though, because it had given up again by 1988, the Nottingham link became part of the A453, and it hasn't been seen since.
If you have a road that is something of a success, you might like to capitalise on its popularity by making it go even further. In most situations this isn't a problem, though it might create an unexpectedly protracted route if you're not careful.
* And this does happen - sometimes. On the two occasions that the A1 has changed its alignment around Newcastle, all the surrounding roads have been renumbered so we can assume the zone boundary moved. But on other occasions, such as when the A3 was rerouted around the Guildford Bypass, the zone boundary has stayed put and resides on the old road through town.
** This happens too. If you go right back to the original numbering scheme, the A5 and A6 bounce off each other in St Albans town centre, meaning that zone 5 begins in London but is squeezed down to nothing before widening out again. These roads are now renumbered but the zones still do this.