You may think you are on an up to 76 Mbps but the 39998 Sync suggests otherwise - unless it is just a fluke. Services marketed as up to 38Mbps or 40/10 or variants thereof will have a max sync of 39998 or 39999. You also have 3dB of downstream margin left which suggest capping.
If you have or could borrow an unlocked HG612 modem you could see what the max achievable rate is and I would guess it would be above 40Mbps not by much but certainly enough to show you are capped or have been put on a 40/10 service.
My advice might be to prove to them you are capped, accept it, and ask for a further reduction in price - the small amount of extra you will get will not be worth the potential difference in cost
M H C
taurus excreta cerebrum vincit