I think there may be two factors at play, first trying to make the overlap a little more plausible (if it can be considered to posess any degree of plausibility at all) because it's based on the idea of those seeing armageddon having heard a "first hand account" of the start of the last days. Since folks in the world at large didn't recognize the signs of the last days (which is why they're not all JWs, or so the story goes) they wouldn't be relating those signs to the next generation (which, apparently would make the next generation part of their generation, but I digress). The annointed, on the other hand, would talk non stop and brag about being present in 1914 to anyone who'd listen. It's therefore more reasonable to think that the annointed would have shared this first hand account so vividly that just by hearing their story you could picture it so well that it was as if you saw the signs yourself.
The other reason is that the contrived way that they define the generation sets a fairly imminent timeline for armageddon. The firsthand account of the events of 1914 has to come from someone who had already recieved their heavenly calling by that time, i.e. they were annointed partakers in 1914. Since in those days, most of the annointed where still coming from new converts (instead of born-ins like now) it's reasonable to assume that 20 would be a pretty good lower bound on the age someone might be at when they first start partaking. So those in the first part of the generation would likely have died off by 1974 around the age of 80. Obviously there might be a few that lasted longer, but this is probably about what most dubs are going to go with. So, the next generation would've had to have been partaking by 1974 in order to be a part of the generation, if you go another 60 years (again, for a total lifespan of 80) you wind up at about 2034 as the upper limit that this system can reach. The general assumption is that it'd happen well before that time due to the unexpected hanging paradox (i.e. if Jehovah waited until 2034, we'd know it must be coming then and Jesus' words about it coming at an hour you don't expect would prove false).
I expect the speculation about 2034 to ramp up in about 10 or 12 years, with subtle emphasis on a parallel with Noah's 120 year ministry (I think this has been mentioned before here as well). They're ripe for a new date setting extravaganza, since by then most of the GB willl probably only barely remember the 1975 fiasco (and what they do remember, they probably have pushed out of their minds).