Two factors are at work.
First, the route plotter does not find the single most efficient route - that would be a "travelling salesman problem " with millions of possible stops, giving millions to the millionth power of possible route solutions to examine - which would challenge even our best supercomputers. So the route plotter takes "shortcuts", by (a) breaking the route up into pieces, and (b) only examining a tiny subset - maybe a few thousand of the millions of possible route options - and picking the best one out of that subset.
Second, the route plotter has an additional calculation it is forced to take into account: your fuel use. It is forced to work on the assumption that we pilots are all idiots, who will refuse to scoop from a star that's sitting right in front of us unless it actually tells us to use our fuel scoop. So it has to calculate "refuelling stops" - it determines when you should reach almost the end of your fuel after a certain amount of jumps, then it calculates the next jump to take you to the nearest refuelling star - even if that's considerably shorter range than your maximum jump, or considerably off your straight-line path.
First, the route plotter does not find the single most efficient route - that would be a "travelling salesman problem " with millions of possible stops, giving millions to the millionth power of possible route solutions to examine - which would challenge even our best supercomputers. So the route plotter takes "shortcuts", by (a) breaking the route up into pieces, and (b) only examining a tiny subset - maybe a few thousand of the millions of possible route options - and picking the best one out of that subset.
Second, the route plotter has an additional calculation it is forced to take into account: your fuel use. It is forced to work on the assumption that we pilots are all idiots, who will refuse to scoop from a star that's sitting right in front of us unless it actually tells us to use our fuel scoop. So it has to calculate "refuelling stops" - it determines when you should reach almost the end of your fuel after a certain amount of jumps, then it calculates the next jump to take you to the nearest refuelling star - even if that's considerably shorter range than your maximum jump, or considerably off your straight-line path.