How this works
NAND datasheets specify a minimum percentage of blocks guaranteed usable (commonly around 97-98%, i.e. up to 2-5% may ship pre-marked bad or fail over the device's life). This reserves that percentage as unusable and shows the resulting usable capacity and block count — a rough planning number for how much of the raw flash to budget for actual data versus bad-block/wear-leveling reserve. Real UBI/UBIFS overhead on top of this is separate and typically another 5-10%.