In general one of the advantages of a standard is, that with adaptors supporting a certain standard, and cables supporting a specific standard, it will work. With that in mind, most IEEE standards tend to be conservative, slightly overenginnered, and will generally work as advertised.
There's nothing stopping a manufacturer from extending the standard to increasespeed - which in this case wasn't always as advertised - or to use a non standard speed or interface. By following a standard, manufacturers ensure that their products, when bought, arn't returned cause they are incompatable.
There's nothing forcing this - standards make sense for everyone involved, since it means all gear conforming to a standard will work together, and you don't need to worry about whether gear from company A and B support different, non-compatable approaches - one reason you can use a ethernet interface (10mbps) with any sort of ethernet cable, and they can co-exist with fast ethernet (100mbps) and gig-e (1gbps) adaptors to an extent.
Its just like networking - there's nothing stopping someone from running an alternate domain system, or replacing HTTP with a different protocol. The standards just make it simpler for everyone involved.