The point on them having had only 6 years pass by. Many possibilities. One, memory fidgeting like smart guy's 'dad's was working on. But affecting the whole population? Not reasonable. Two, the year the kids are told is not actually the year, but an earlier number(ex. They grew up believing it's 2011 when it's rly 2019). An additional point that could fit in either regardless is that the earth Oneesan has actually been hibernating for a lot longer than they thought.