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It is astonishingly tiny that there is such a little variation between two days in a year!
When looking at entire seasons, what are the COPs?

Assuming you have a sensoCOMFORT or have enabled it in the MyVaillant app, there has probably been a firmware update or two since last winter.
Since we haven't entered cold weather yet, my Vaillant aroTHERM Plus's performance varies greatly even on days that seem identical. However, they differ significantly in that some have significantly lower ambient temperatures due to significantly stronger winds than others.

For how long has it been in the shop for maintenance?

It seems to communicate with the system via the sensoCOMFORT rather than the control module, so you might want to double-check that the batteries in your external thermostat are fully charged and that it is functioning properly.
If your system uses weather compensation with offset through the internal thermostat (also known as Room Temp Mod by Vaillant), make sure there is no dust or other obstruction on the thermostat to guarantee accurate temperature readings. Otherwise, if your system uses pure weather compensation, no need to do anything special.

This may sound foolish, but have you considered turning off the power to the external unit, controller, gateway, and internal thermostat? There is a lot of technological complexity involved with these things nowadays, and things can become out of whack if some parts have been reset while others have not.

It has been extremely windy where I live on the south coast, which is relatively close to the sea, for quite some time now (thankfully, the wind has died down today). In comparison to your system, mine used 31.1 kWh between October 19th and October 25th, with 27.5 kWh going toward heating and 3.6 kWh toward hot water. The energy efficiency rating was 4.59. I have my house set to a temperature of 21 degrees Celsius from 4:30 in the afternoon to 11:30 at nite with a 19 degree setback. Every day, except for Monday, when the legionella cycle is active, the hot water is set to 48 degrees. To automate the change to 55 degrees on Monday, I used a Home Assistant integration, since the system only lets you lower the target temperature to 60 degrees when the legionella cycle is active.

On the 25th, the total energy consumption was 5.0 kWh, with 4.1 kWh going toward heating and 0.9 kWh going toward hot water. The efficiency was 4.6. Throughout the day, the average outside temperature was 10.5–15.3 degrees Celsius, with a 5–degree "feels like" temperature.
The wind was fairly strong, reaching 43 mph on occasion and averaging around 21 mph.