Lifting up on the throttle doesn't "screw" with the compter's logic. The computer returns absolute throttle control to the driver after it has competed shifting, even if the driver has moved the pedal during the shift, and simultaneously re-engages the clutch in a variable manner depending on that pedal position. This allows the driver to finess the re-engagement of the clutch as desired after the shift, whether it may be to snap ahead faster, maintain steady speed, or decelerate. It is not necessary, but using this technique can sometimes adjust the smoothness & speed of the shift & engagement to suit individual driving style or conditions.
Basically, I find that post-shift clutch re-engagement is calibrated quicker with added throttle, and slower with less throttle.