Possible sources of constant STM32L4 ADC offset error

Solution 1:

The STM32L432KC Nucleo board does not come with a high precision reference. As you noted, it's merely the 3.3V LDO powering the analog regions of the device. This is likely where your error is coming from as it's a relatively cheap, inaccurate part generating that 3.3V. Calibration helps with offset and gain errors, and can lead to a more accurate measurement, but if you use the typical conversion approach from LSB to absolute value (and you're looking for that absolute value), an incorrect reference can give you that error right back. Page 517 of the family reference shows how to implement this in software.

On top of this approach, you can attempt to measure the absolute voltage of your reference through an internal reference using the process outlined on page 582 of the family reference. If you combine this approach to get a good grasp of what Vref+ is from the LDO, combined with proper calibration, you'll likely be able to reduce your offset error, as well as the slight gain error you were showing.

Resources: STM32L4 Family Reference, Best ADC Accuracy Procedures