Your extruder system has a finite amount of pushing force. You want as much as possible of that pushing force to be used to generate pressure in your HotEnd, as it's what causes the molten filament to actually extrude from the print-head (crucial to 3D printing!).The friction of filament sliding its way from the filament drive to the HotEnd saps away some of this force, leaving less force to create useful pressure. When we use a bowden system the filament has to travel on a longer and more convoluted path, and so we get higher losses to friction. While standard PTFE is slippery enough to mitigate this effect, it does not eliminate it.
Another error source in extrusion systems is the compression/buckling of the filament. As we apply pressure to the filament in order to extrude it will bend a little (but a little is all it takes) between the extruder drive wheel and the HotEnd melt zone. Because of this spring-like compression we can advance the filament by 1mm at the drive wheels, but 1mm's worth of filament will not immediately exit the nozzle in perfect synchronicity. In a bowden system this filament-spring is softer due to its much increased-length. The subsequent deformation under force results in extruder motor speed changes taking longer to be reflected at the nozzle, and the errors being greater in magnitude.
Capricorn XS bowden tubing helps with both of these issues. It is made of pure PTFE that has been treated with a special additive to reduce friction. It is also made to highly precise standards, with an extremely tight internal diameter to improve filament constraint between the extruder and the HotEnd.