Procurement creep, surely?
I recall a confident announcement in 1996, when were told by MoD that the VC10 and TriStar would be out of service "in the next 10 years", to be replaced by some 25-30 'MRTT' aircraft (A310MRTT or B767-200ER were the favourites).
Mind you, we were also told that the in-service date for 25 x 'FLA' (which became A400M) was 2004....
Now 14 years later, the poor old VC10s and TriStars are still struggling on, there are no A400Ms in service yet (and their numbers have been cut), the '25-30' MRTT have become 14, of which only 9 will be in regular use....
And we thought that it was bad when they binned 99 and 511 all those years ago...
Hope the poor buggers in the Great Sandpit get home for Christmas!