Often the bigger difference is just that fiber never goes in a straight line, even if it’s going to the right city. All that pesky geography gets in the way and makes the path longer.
As far as I can tell it almost exclusively follows the existing roads in Europe. Probably an easier way to secure rights in one go like rails used to be for telco lines.
If you were smart about it then you wouldn’t go up in the mountains in the first place. To get the lowest possible latency you want a straight shot, so obviously you should just bore your way through at the level of the plains on either side.