Why could an early Egyptian daylight hour be longer in summer than in winter?

Answer: Sunlight was split into shares

Water clocks ran faster — Water clocks were real ancient tools, especially useful when shadows were unavailable. But water flow is not why daylight hours changed length. The seasonal stretch came from dividing the changing span between sunrise and sunset.

Shadows moved more slowly — Shadow movement is part of how sundials worked, so this is a tempting guess. But the hour length changed because the whole sunrise-to-sunset span was longer or shorter. The scale was a fraction of daylight, not a speedometer for shadows.

Sunlight was split into shares ✓ — Correct. When sunrise-to-sunset is divided into 12 shares, each daylight hour grows longer in summer and shorter in winter. Early hours were portions of daylight or darkness, not fixed 60-minute containers.