A very very interesting question Lalito:
Read this fragment of Wikipedia, and after that my explanation:
Linguistically speaking, 'Castilian' means 'Spanish', as it is the medieval Castilian language that spread across Spain and became the national language known virtually always as 'Spanish', in English. But in Spanish itself, the term castellano (Castilian) is often used to refer to the language, at the expense of the term español (Spanish). It seems that awareness of the term 'Castilian' is growing in English, and even more so in French.
"To understand how two terms can refer to the same language, imagine that the English language were sometimes called 'English' after the historical nation whose language it is, but also sometimes 'British' after the modern state of which it is the official language. Imagine then that Britain had an empire of colonies in the Americas. How would they then refer to the language? What about the speakers of Welsh and other non-English languages spoken in Britain? This is almost exactly the situation with Spain and its historical heart, Castile (Castilla)."
Yeah, spanish and Catiliano are the same. the difference is in the historical concept. From What I learn at school is that at America people speak Castilian and at Spain, Spanish. The reazon: When conquers came to America they were from Spain, but they were people of low class, I mean not princes, not richs, people like begers, prostitutes, murders, etc, because imagine who in that time wanted to come to a new world they don´t know?, only people who likes the adventure, that people came from the provinces of Spain, not from the cities, most of them from Castilia. In America, after some years here and after they mixed with indians, the languages got a transformation, then spanish at America is a mixture from Spanish and "indian voices", of course after indians learn spanish, and then people began to call the new language "Castilian", because they hear conquers came from Castilia.