Factors affecting precision
We like to imagine that when we make an object in Blender that everything will just be exactly the size we specified. But this is not always the case. 3D printers exist in the real world and there are a lot of things that can affect how precise your object is:
The size of the object
How well the printer is set up
The quality of the printer to begin with
Any sloppiness in the system
Whether the printer controls the X, Y, and Z dimensions equally well
Printing speed affects how much extruded material is laid down at any given spot
How recently the printer was calibrated, and how well it was calibrated
How consistent the extrusion and material flow is
The ability of the software to control the system
How much flex there is in the printer
The ambient temperature of the room
These things can result in objects being stretched or squashed, holes not being round, objects that should fit not fitting, straight lines being wavy, gaps between the layers, and more. The burden of keeping these things under control falls to the person with the 3D printer, but it's important to keep in mind when you are designing your objects.
