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Showers soak more then pools
(hackertalks.com)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
2 main factors here.
Time between getting out and wrapping yourself in a towel.
How much of your body is actually wetted.
The first means you have time to drop dry, before you grab your towel. It's only 10s or so, but you can shed a lot of water in that time.
The second matters, since you tend to keep your head out of the water in the pool, but soak it in the shower. Wet hair can hold a lot of water. Even if you don't dry your hair, the water will run down of the rest of you, as you dry yourself off.
Another point is that the air after a shower is VERY humid, compared to being under the sun after swimming
OP mentions fully submerged so the hair should not matter.
But even then, it's very likely that your head left the water long before the rest of you.