There is a bug with backpacks involving the less carry weight for specific types perk.
Ex. Say you have two backpacks that have 7% reduction to mining items (ores, mining pick, etc…)
If you equip one of the backpacks you will see your weight go down, if you swap that backpack with the other one it will go down even more. You can keep swapping them repeatedly and the weight of those items will keep going down until they are less than 1.
I believe this affects your fast travel cost, but I’m not sure you are truly getting the weight reduction.
Rough example, starting out with 799 weight and 800 max weight and carrying 600 lbs of mining items. Swap out the backpacks until you get under 201 weight displayed. Now if you pickup more items your character will get bugged and stutter when moving, can’t cast, etc…
These are rough examples, but you get the idea. Just pick up two backpacks with the same reduction type and swap them repeatedly, you’ll see the problem…