One thing worth noting is that we are assuming a straight line (bird's eye view) of the trip. There are, no doubt, obstacles in the way that might cause the ship to slow in particular areas. Exam,s include dense asteroid fields (as opposed to what we have in our syste with 100,000s of thousands of moles beTween rocks), blac holes (which you really need to avoid for obvious reasons), nebula (which require either sub light speed or using warp speed to go around the nebula, neutral zones, etc. see Star Trek II for an example of this when, during the Kobayashi Masu, she orders Sulu to change course. No straight line is available from point a to point b. she has to take a parabolic course.
My point is that perhaps we are focusing too much on the speed and not on the obstacles between the ship and the final destination. YMMV.