post is in the "Wifi" group.
If you're actually talking about a copper cable, speed is a derivative
a) both sides of the link, i.e. the network card and switch (on the principle of the "lowest common denominator"). Limited 100mbit network cards are from the age of dinosaurs. The fairy tells you that the switch limits you.
b) cabling quality.
c) settings
In the wiring of dubious quality, I often limit the speed to 100mbit, I prefer to have a stable 100, than 1Gb with humor and spells.
EDIT, of course, still words from the borderline of marketing providers, the transmission has mark-ups, firstly, secondly, written in papers, etc ... kilo / mega / giga in binary terms is 1024, 1048576, 1073741824, in decimal 1000, 10000000, guess which use marketers