The issue here is that some crews will take their booked break as their are entitled to do, but others will merrily go without anything at all if the circumstances are such that it reduces delays (often if it is their 'going home' train, i.e. last train of the shift). Control/resources staff are not shy of asking people to move, reduce or give up their breaks, sometimes in return for an overtime payment, which I personally find rather disagreeable. The variations in individuals' attitudes can mean that sometimes a train will be delayed, and sometimes it won't be. Station staff might assume that because a certain service turned around in five minutes last time it was delayed, the same will happen again this time, but the crew this time may wish to take their break or indeed it may even be a different crew on a different diagram. Equally, some crew memebers are a tad more proactive in communicating than others, and some may not reveal that they will be leaving late until they arrive, whereas their more helpful colleagues will have already phoned control. None of that changes the basic fact that train crews need a break (guards as well as drivers, our cock-ups can kill people too), but abuse and/or ignorance of that is not rare. The obvious comparison with professional road vehicle drivers is often given, the difference being that their restrictions are legally enforceable whereas those on the railway aren't - a significant failing in my opinion.
As for the public information, yes it is of course deeply irritating when a stated departure time is suddenly scuppered for no apparent reason. However, bear in mind that the railway tends to work on the 'root cause' where delays are concerned - so if the crew are late arriving because of a level crossing failure or whatever, that will be the given cause of any resulting delays for the rest of the day. Hence the slightly bizarre explanation 'congestion caused by an earlier delay...' - when something has caused widespread problems that reason will be the stated cause of residual disruption during service recovery. Stating a miriad of other individual reasons for lots of different services gives the impression that there are hundreds of isolated delays, when in fact the whole show is actually up the swanny because of one particular incident.