Simply - The generation who saw 1914 passed away so the society expanded the term 'that generation' to include people who were alive when the generation who saw 1914 were alive.
That simple explanation would get you called apostate though, so to a JW on the edge:
Since society teaches the last days began in 1914 and Jesus said 'this generation will by no means pass away', our current understanding of the term generation encompasses those who saw 1914 and those whose lives overlapped with them. So if, for example you grandmother was born in 1912 and lived to see your daughter (her great grandaughter) who was born in 1993, they are deemed to be part of the same ovrerlapping generation.