This is a common trick of bus operators to inconvenience passengers in order to meet TfL targets. The timing points at which performance is measured are at busy interchanges, but the measure of whether they are early or not is done by arrival time rather than departure time. I have several times missed connections to trains or other buses because of this practice. TfL should realise that nobody minds if a bus arrives at its destination early. (Leaving early is another matter entirely of course, but they don't measure that).