There are differences in vocabulary, accent, second-person verb conjugation preference, idiomatic expressions, heritage and culture. In other words, the differences are significant from country to country. A person from Spain can be understood in Mexico, and vice versa, but not without some difficulty and ear training. It can be likened to how well you understand a person from England who uses a thick British accent and all those words that we think mean something else.
From what I can tell, your textbook is teaching Latin American Spanish.
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