I think the key here is that some of the anointed are supposed to be alive when Armageddon comes.
Because of this, they have had to change the doctrine several times as time has rolled on. In the beginning, it all fit together quite nicely; The 'generation' Jesus was talking about was understood to simply mean all people who were old enough to 'remember' 1914 and the things that happened then. (That the 6000 years since Adam was coming up in 1975 seemed to gel nicely with this doctrine, and all was well).
Unfortunately, the years went by and nothing happened, so they conceded and said "OK, so you don't have to remember what happened in 1914 - let's say it's all people who were born in 1914". But eventually, again, the years went by and nothing happened.
Come 1994, and it's 80 years since 1914. In other words, even those born in 1914 were 80 years old. "Man's age shall be 70, with much strength 80" and whatnot.
So 1995 rolled in, and with it came the (now pretty obvious) change in 'light': The 'generation' was now suddenly not all the people that had been born in 1914; it was all people who would see the composite sign of the coming of Christ - meaning it could span several of what we usually consider to be 'generations' of people.
However - ten more years (and then some) passed, and even this wouldn't do anymore. Because the anointed are really becoming old - so much so that they consist of 40% dust by now.
So - we need another change. Actually - we need several changes:
At the start of 2007, 'new light' revealed that heaven is 'open again' - the gathering did not stop in 1935 after all, and the newly anointed (those who decide to eat and drink) were not necessarily just substitutes for anointed who had 'fallen away', but actually newly anointed.
This was section one in the rocket they had launched. This section made it possible to postpone Armageddon for a few more years, because now - even if all the old geezers die of old age, there will still be anointed on earth. The next stage in this 'rocket', was the new announcement (not yet "official") that the 'generation' in fact isn't people in general at all, but only the anointed.
And voilà! We have gone from a 'locked' doctrine with a definite expiration date, to a doctrine that can go on and on and on and on.... and on. And it's been done in several more or less 'smooth' steps, so as to not upset the average JW too much. After all - it's wonderful that 'we' get all this new light... Only negative side to it is that they may expect a huge increase in anointed in the next couple of years, but I'm sure they'll "handle" it.
