[Here](https://www.youtube.com/watch?v=Ri_4dDvcZeM)'s a great video by Steve Mould on Tom Scott's channel which explains it.
Essentialy it's because the viscosity (thickness) of water changes at different temperatures, changing how it splashes as it's poured.
To add to this, when running the water for my shower I've learned I don't need to keep touching the water to check if it's warmed up yet.
The sound the flowing water makes while going through the pipes changes slightly - decreases - as the water running through it warms.
As soon I hear the dip in frequency I know I can tolerate the water on my skin.