If there’s congestion, unlikely, then you might not get the full speed you expect. Also if there are corrupt or lost packets then these will need to be retransmitted if you are using TCP for example, so that will reduce the speed you’re getting across the internet.
Referring to what tubaman said about overheads, if you are on the G.INP retx high setting that will take away about 8% of your speed due to wasteful overhead, so you won’t be getting the full speed associated with your sync rate. Normally the true speed you get is about 97% of your sync rate. By ‘speed’, I mean here, to be exact, IP PDU rate. Overhead due to IP headers, and usually TCP headers too, will need to be subtracted as well if you want to get the file transfer rate, ie TCP payload rate.