Why are visas in Britain divided into visas and entry clearance?

"Entry Permit" is the official term for the visa you need to enter the UK at first. This visa is

Usually called "entry stamp", "entry visa", "entry permit" and "entry permit stamp"

Or "entry permit visa"

In other words, EC is just one of the "visas". It is not clear why there is such a difference. One of the reasons may be to count the number of net immigrants.