They generally don’t go to Brighton in traffic any more and can be moved between St Leonard’s and Selhurst avoiding Brighton, but it could be any reason. For example they could only have a driver available to take it to Brighton and then another driver to take it forward to Selhurst the following day. Due to strikes/lesser timetables for whatever reason, St Leonard’s could be busier than usual and SE don’t have anywhere to stable a 375, so SN have shifted a 171 to make room. Again, not saying these are the reason but just a rough example of why these things could happen.