5miles,
A bit more background for you..... Slotted traffic is straightforward to predict (and thus flow), given that there is a 15 minute window for its airborne time. Vagaries we are all aware of, such as; short cuts/direct routes; weather; different speed profiles on departure etc etc make the current flow system a blunt instrument.
The problem comes when aircraft going through, say, CLN sector between 0900 and 1000 are regulated, but the aircraft in the hour 0800-0900 are not regulated, as the demand is well below TSF. Now, if a few of the aircraft scheduled 0800-0900 are running late, but the EOBT is not kept up to date, they could get airborne to fly through CLN between 0900-1000 but CFMU system is 'not aware' of them, thus causing (possibly) an overload.
By keeping the EOBT up to date, and not allowing any a/c to start after EOBT+15, then those aircraft running late will be now be given slot times and will be factored in to the flow for that hour.
Measures such as the 250kt trial in the TMA was partly to increase the accuracy of flight time estimates to aid flow.