Actual transfer speed over a 100Mbit network

[#1]

I ask this because I have just run the monthly Acronis backup on my better half's PC (I do it manually from a boot disk) and am currently copying the file over to my PC via a 100Mbit network and the speed I am achieving is approximately 77Mbit
Is this the actual limit or is something mucking it up
If it is the actual limit where has the other 23Mbit gone to
Edit - speed has now increased to about 83Mbit with upload 1.7Mbit

[#2]

it's always less, due to header and footer (or trailer) but it's more likely to be due to the speeds of the disks r/w and possibly the nics used.