Even though I have a full tank (300FF), setting fuel consumption to 100% shows only 182 units available.
Sounds normal to me. It doesn’t show 182 available, your engine just isn’t capable of burning more.
Since no one with any actual knowledge is going to explain this, I’ll have an uneducated guess at it myself. Ok, I’m guessing you can’t use FTL in atmosphere, and the amount of fuel you put in doesn’t effect the speed at which the FTL engine runs, only for how long it will run (FTL must have only one speed, on or off). So what’s happening here is the distance between these two planets must be far enough for FTL to be used, but not far enough for all the available fuel to be burnt. But that’s only a guess.
The FTL drive is only used for jumps between star systems. A reactor has a certain power rating and the fuel used in the flight is dependent on that rating, the distance and the factor provided when planning the flight. On short distances it might not be possible to use up all of the available fuel.