Why one extra battleship changes American basing

Within the proposed tonnage allowance, 40,000-ton ships could give the United States twenty-one battleships instead of twenty. The twenty-ship alternative suggests five four-ship divisions, with three in the Pacific and two in the Atlantic. Twenty-one ships instead allow six three-ship divisions and three flagships, forming seven-ship squadrons at Hawaii, San Diego and on the Atlantic coast. Fourteen ships would then be available in the Pacific, matching the hypothetical Japanese fleet, while seven faced Britain's Atlantic force. A twenty-ship fleet could use a similar distribution, but one squadron would have only six ships. The extra hull changes the local numerical balance without increasing the national tonnage allowance.