Because the nbbo was built around round lots, orders for 1 to 99 shares could sit at better prices without appearing in the national quote at all. In high-priced stocks, where a hundred shares is a large sum, that hid a meaningful share of real liquidity.
Rule changes have been moving toward publishing odd-lot information and, for some securities, recognising better-priced odd lots, but the legacy gap is why displayed and actual best prices can differ.
Example: a $900 stock shows an NBBO of 899.50 / 900.50. Behind it sit odd-lot orders at 899.90 and 900.10 for 40 shares each. A buyer of 40 shares could pay 900.10 rather than 900.50, saving $16, if their router looks past round lots.
Related: odd-lot-order, round-lot, nbbo, tick-size-regime