The official guidelines as towards duration are given here:
http://www.bbc.co.uk/commissioning/delivery/durations/
WebbieOTW is quite correct in that it says that "The physical programme you deliver must conform to the actual duration requirements shown below" (for a 45 minute slot, the actual duration is 44 minutes). Further down that page it also says
"Programme tapes must be delivered at the correct duration - this is essential in order to ensure that schedules can be made to work to time and that key juntions are met (e.g. with live News).
Programmes delivered which are not at the correct duration will be returned to the programme department or company for re-editing to the correct length"
However that page does go on to say that:
"Occasionally the actual duration of your programme may need to vary from the guidelines above - in this case your commissioning executive or CBBC on-air will advise you of the actual duration to which you should deliver."
and also:
"Any variation to the agreed duration must be formally agreed (by writing or email) as specified here."
As DW has occasionally been longer than 44 minutes then either
a) its agreed slot is longer than 45 minutes (say 50 minutes), but the schedulers try to squeeze it into a 45 minute slot anyway
or
b) its agreed slot is normally 45 minutes but the commissioning executive has sometimes indicated a longer episode is needed
or
c) the production team has had trouble cutting an episode to its required 44 minutes and have managed to negotiate (and get written agreement for) a longer slot - presumedly the fact that DW is seen as one of the flagship Saturday night programmes makes this negotiation easier than would normally be the case.
Matthew