How do temperature fluctuations occur in fresh water?

Good day.
I hope I am in the right forum.

Who can explain to me how temperature fluctuations can occur in fresh water?
The fresh water runs with a target temperature of 60 degrees. Circulation is 55 degrees.
It receives 65 degrees from the buffer tank (limited by a mixer), which operates at about 70 degrees.
The fresh water then produces about 60 degrees at the apartment outlet.
From time to time, however, it happens that the hot water fluctuates slightly while showering.
So it does not stay constant as set on the single-lever mixer.
How can this phenomenon be explained or eliminated?
Is it because if too many people need hot water at the same time, the fresh water does not reach these 60 degrees?
Or maybe because the withdrawal at that time is even too low?

Thank you

Spontaneous fluctuations on the single-lever mixer almost certainly come from pressure fluctuations in the hot or cold water line.

Pressure fluctuation == flow rate at the mixer changes == final temperature changes

Spontaneous fluctuations in the single-lever mixer almost certainly come from pressure fluctuations in the hot or cold water line.

Pressure fluctuation == flow rate at the mixer changes == final temperature changes

Thank you.
But where do such pressure fluctuations come from? We hardly ever had this problem before.

What does vorher mean?

Cause: Toilet flush, dishwasher, ... (everything that has a solenoid valve), possibly other dispensing points if the pipe diameters are tightly dimensioned

The only sensible explanation has already been given to you by . The pressure fluctuates. Often triggered by someone else using water. For example, flushing the toilet etc. Fluctuations in the primary network, i.e. the supplier, are also conceivable. And what does it mean, the problem did not exist "before". What was before?

What does "before" mean?

Cause: toilet flush, dishwasher, ... (anything that has a solenoid valve), possibly other tapping points, if the pipe cross-sections are narrowly dimensioned

All right. So such fluctuations are not due to insufficient temperatures in the buffer tank. I thought that producing domestic hot water at around 60 degrees would not be enough with 65 degrees, and that this would cause these fluctuations.
I would have then thought to lower the domestic hot water by 2 degrees or raise the buffer flow by 2 degrees...