Agree with the E500. But for the E400 it is a bit of a different story. When launched in 2005, the whole vehicle was marketed as the E400, but the chassis still retained the "Trident" name.
So as far as I understand, it's ADL who markets both types very different - E500 is called a E500, body or chassis or the whole thing, as it wants to imply to the customer that it is a brand new product. In the case of the E400, the chassis is still marketed as Trident because the 2-axle Trident has been a succesful and renowned product in the UK. It makes more sense to retain the name for 1. marketing purpose and 2. its availability on other (ok, just one) bodywork.
It's all marketing ploy. ADL's predecessor has done this before with the Lance when it got double decked. Despite the chassis was essentially the same, the double deck version was called the Arrow instead.
