Incorrect inner dimensions of 3D prints with Ender 3
There are 3 effects at work, and you misread your micrometer: the measurement is 19.35 in the picture.
You have a little lip
There's a little lip at the top and bottom of the print. You'd need to clean that up with a sharp knife or sandpaper. That is the biggest part of the error you measure.
Movement errors accumulate
Errors also collect on the center of holes due to the order in which walls are usually placed, resulting in outer walls having the correct diameters but inner holes having a small offset.
Plastic shrinks when it cools
A smaller part of the error is the plastic shrinking as it cools, but that can be compensated for by the slicer - if your printer allows for it: Under Materials, there is a Shrinkage Ratio setting.
There's compensation for that in Cura
The option is under Shell and called Hole Horizontal Expansion. Setting that value to 0.6 mm to 0.7 mm should solve the hole sizing error.