Agreed! The fabric of space-time is variable. It could have been made more clear that could account for inefficiency of the engines. I think there may have been an episode or two in Voyager that stated this, but weren't they mostly cruising at Warp 6 or 7 most of the show because the ship couldn't go past Warp 8 all the time? So this could account for some of the 70 year trip. Another thing is that they couldn't always go in a straight line to the Alpha quadrant, let alone straight to Earth. There are bound to be detours, not to mention that the galaxy is 3 dimensional. Sometimes, they had to travel up and down the Z axis of the galaxy, not just the vector straight to earth, right? That is bound to add years, too.