WATER TEMP GAUGE

I have noticed this  (and previous and maybe 2)  winters before that when outside temp drops down to 5-6 degrees the water temp gauge once reached 90, it can then start dropping when driving for example a downhill or when going a slight incline downhill with 6th and say not over 2000-2500 rpm and when the engine runs very low..

Engine 1.4 TSI.

NO noticed any problems though but even in hot summer days when the gauge shown 90 in water and not exceeding not even one degree with outside 38-40 and climatronic running...very stable!

But what is happening in cold winter days and does not stay 90?

Does thermostat stay closed so that it recycles coolant more often than should?

Opinions appreciated.