The CTOT, although a definitive time for planning purposes, is acutally a window of CTOT-5 mins to CTOT+10 mins. With this in mind it's easy to see that of, say, twenty aircraft inbound to LHR with slot times spread out over 30 minutes, if ten aircraft get airborne at the back end of the slot window, and another ten get airborne a the CTOT-5 end, you might end up with 20 a/c arriving at the stacks in a very short time.
Also bear in mind that the landing rate at LHR is dependent on the traffic mix. If there's a declared capactiy of 42 per hour, and a lot of them are heavies, you might only land 38 in the hour. All sorts of things come in to play. ATFM as it is practiced is a very blunt tool. There is a lot of work going on behind the scenes to try and sharpen it.