Why post-Hawkins cruisers started at 12,000 tons

Why did the Royal Navy prefer a 12,000-to-13,000-ton post-Hawkins cruiser instead of starting at 15,500 tons?

Design growth was expected: starting near 12,000 or 13,000 tons could yield a finished ship near 14,500 or 15,000 tons after armor, ammunition, fuel, and crew needs accumulated.

The 12,000-to-13,000-ton figure is an initial design target. A finished cruiser could reach about 14,500 or 15,000 tons as its requirements developed. Starting near 15,000 tons could instead produce an 18,000-ton ship. Armor, additional ammunition, more fuel, and extra crew space each offered an attractive improvement for a small increase in displacement. Hawkins provides the comparison for how those individual additions accumulate.