VLd1977... You are absolutely right. ATC often has to change the take-off order for a variety of reasons which go against the first-come-first-served principle. First at the holding point might have 5 minutes to wait for his en-route slot so 2-3 others may go ahead. In the approach phase, aircraft types are juggled to achieve the best landing rate which may not be on f-c-f-s basis.