Now when testing with the exact same settings once again, my sens is a 370, and on the next restart a perfect 360. I believe its the dreadful Corsair sofware doing its thing. My new mouse comes on monday, so I'll test with that and see if the problem lies there. Thank you for your attempts at helping however! I'll update if the same problem pops up with the new mouse once again.
EDIT: Nope. Went back to being completely wrong The issue is seriously only in fortntie, and it just randomly fixed-then-messed back up ingame. Im re-installing to see if it works this time. & I don't have a ruler so I can't test but, in OW it takes up my entire mousepad which should be about 45cm (from left to right) while in fortnite it takes my entire mousepad + around 15% of it (so I have to do one swipe from left to right, then another swipe from left to a quarter of it, so, 11.25cm, ish?) Around 56.25cm...ish. My mousepad is huge spent all my money on this big mousepad rather than getting a ruler to measure!
EDIT #2: I've found the problem! Turns out having Vsync off heavily fucks with the sensitivity. After turning it on, perfect 360! Seems to be the oone causing the problem this time.
EDIT #3: Okay this is just extremely odd. Apparently, switching VSYNC back and forth (4-5 times for me) fixes the sensitivity. My graphic settings also reset, so this is EXTREMELY odd.