Two pools at the same thermometer reading can feel completely different depending on what’s dissolved in the water. Higher mineral concentration slows heat loss from skin, so a 39°C pool with a heavy magnesium load can feel a degree or two warmer than a 39°C pool with very little dissolved mineral.
This is part of why we test and adjust mineral concentration separately from temperature, rather than treating the thermometer as the whole story. Our water systems lead checks both weekly, and adjusts flow rate from the source spring rather than adding anything artificially.
It’s a small distinction, but it’s the reason six pools at Thermae can share one spring and still feel like six genuinely different rituals.