That's not surprising. Italian is full of dialects and the Italian that gets passed down from one generation to another would tend to deteriorate over time. What little I know my Dad had a hard time understanding my pronunciation or the context in which I used it. By the way, people in Milan or venice not only use somewhat different grammar and vocabularly, but their pronunciation is different too.

The same can be true of other languages. For example, I have many students who are Hispanic and speak Sapnish. But, some of them cannot readily understand certain phrases, vocabulary of other Spanish speakers. In fact, I have not found a Spanish speaking person yet who can translate some of the Spanish spoken in the film Carlito's Way.