've found the fares displayed during the course of a journey aren't always right. If, for example you travel to London Bridge and exit through the barrier, you will get the fare for that journey. If you then continue on the Tube, you will get another fare when you touch out. But I don't think that takes account of the previous part of the journey. If you then get a statement, it will give the fare for the total journey and quote the first part of the journey (eg Forest Hill to London Bridge) as having a cost of £0.00. So far, the fares on the statement have been right for me, even if it takes a while to understand.