I am by no means an expert on this, but have a fairly good understanding of ASDO, so I'll try and explain it...
The 'issue' here is with the type of Automatic Selective Door Operating (ASDO) system. The Class 375s were the first units to have full ASDO, with the system known as Hima-Sella Tracklink 1. Simplified, this system uses a GPS (technically GNSS but GPS is a more familiar term to many) receiver in the leading cab which is linked to a database of every station and their door openings. Each station is defined by a central point, and then has a 300m geofence around it - which, when entered, confirms the station and doors to open to the train.
Where Tracklink 1 fails is that it treats every platform as the same length, and for safety this is understandably assumed to be the shortest platform. When developed, there was no way to identify individual platforms and GPS wasn't accurate or reliable enough to be used, and so the same length was assumed for all. For example, if we take the relevant platforms at Victoria, from shortest to longest (below), you can see the shortest can only just hold 9 carriages. Now, when the service leaves the previous station, there's no absolutely firm way of knowing which platform it will arrive into and the system treats them all as being 188m long, just in case it is routed into platform 3.
Platform 3 - 188m (9 carriages of 375 stock is around 182m depending on the subclass and combinations)
Platform 4 - 201m
Platform 8 - 221m
Platform 6 - 245m
Platform 5 - 247m
Platform 1 - 270m
Platform 7 - 286m
Platform 2 - 359m
This might all make more sense when you compare it to Tracklink 2, on Class 377s for example. With this 'newer' (relatively newer, it's now fairly old!) version there are also added readers, antennas and transmitter loops in the four foot to identify specific platforms. The train will still assume the shortest platform length, but if the train goes over a transmitter which identifies it is approaching a longer platform, then the number of carriages on which doors will open increases.
Now, of course, coming to the reality of things... Firstly, I have never been on a 12 car 375 that has been too long for a platform at Victoria - it's very rare for it to happen. When a 12 car 375 arrives at Victoria platform 3, only the doors on the front 9 vehicles will automatically open. If one arrives into platform 2, then again, only the doors on the front 9 vehicles will automatically open when the driver releases the doors. A manual selection can then be performed on the 375's MITRAC system (train control and management) if the driver knows and is absolutely sure the platform can hold 12 carriages. This takes slightly longer, and is noticeable if you're in the rear X carriages awaiting the manual selection to release the doors, but is only around a 10 second wait. Again though, in reality, 12 car trains mainly work commuter services which have regular passengers. Those in the rear carriages know that the doors will probably open, contrary to the announcement so don't all rush forward. But, there will be one day when, due to disruption or something out of course, the train pulls into platform 3, the doors on the rear 3 carriages won't open, passengers will complain... but they'll have been told!
How could it be fixed?
Well, personally musing, I guess you could remove the announcement altogether and instead issue a briefing to guards to say something along the lines of... "Depending on the platform we are allocated at Victoria, the doors on the rear 3 carriages may not open due to some short platforms, but I'll let you know just before we arrive if this will be the case as at the moment I do not know which platform we will arrive into" and then just before arrival, the guard could read the route/platform indicator and make a further announcement to confirm or correct how many carriages will open. But, this is fairly long winded, confusing, complicated and it can't be guaranteed that a guard will make the announcement every single time (and then there could be confusion if there's no announcement at all).
