I have a friend, that has Comcast, that was at one time using an older router that I had given or sold to him a long, long time ago, and when he hooked it up, his internet plan was bumped up again, and was higher than 100Mbit/sec (120Mbit/sec or so), and his router had 10/100 ports on it, so his speedtests were capped at 94-95Mbit/sec. That's the kind of overhead you'll see.
76Mbit/sec, that could be cable "speed lag" like Jack said. Or maybe something else is amiss, or your router is in fact older, and can't actual route 100Mbit/sec WAN-to-LAN, even though it has 10/100 ports on it.
Get a gigabit AC-compatible router if that's the case. (If router is more than 3 years old, and not AC-capable.)
Edit: Here's a paragraph from a review of an Asus AC1200 router (which I just picked up):
Got a new ASUS AC1200, followed every instruction, and updated the firmware. When the process was completed I check my modem speed (modem connected to the PC with a network cable), my modem speed was around 120 Mbps, next I connected the PC to the No. 1 LAN port in the router and tested the speed again all I got was a max of 80 Mpbs, next I check the WIFI speed (wireless connection) I got the same as from the router LAN port, around 80 Mbps max. This is a decrease of around 33% of the modem speed. I got in contact with ASUS customer support, they were very pleasant, and we spent over 2 hours over the telephone. I changed cables, channels, reset 3 times, and the results were always the same. ASUS suggested to send the router for repairs, however since this is a brand new product I decided to return the router.
This is my personal experience, the product maybe great, but cost me half a day’s work for nothing.
So, it seems with some routers, the overhead is higher. So it could be router overhead, especially if you have that same router.
Still, given my experience with the Netgear router, that it was only a 5Mbit/sec deficit, then I don't know.