Because this voltage is somekind of neutral voltage for the chemistry used and cells with this voltage age the less. There are two aging effects, if I recall right, one happen in lower voltages and one in upper voltage. Indicated voltage value is a compromise, sweet spot for longetivity.Many recommend, when storing cells for long periods, only charging them to ~3.7V-3.85V. I haven't heard an explanation for why, tho. But it sounds like OP is not storing cells so much as preparing them to be ready for use when needed, in a year if it takes that long.