Probably the same reasons there are misunderstandings in every area of life. I offer a few possibilities, as follow:
1, Human nature. Ever since the tower of Babel there have been misunderstandings, which are primarily rooted in our own sinful nature,
2. No common basis. Every country has its own laws which serve as the common basis for their legal system. Try applying one set of laws, as in Sharia law, in another country, say Great Britain, and you wil have misunderstandings. If Christians have as their sole basis for understanding the Bible, then there would be fewer misunderstandings. As it is, most, if not all, denominations toss in their own traditions and extrabiblical materials to the Bible, resulting in many misunderstandings.
3. Linguistics. Grace, for example, may mean something entirely different to another person based on two different definitions of the word. Standard definitions are difficult to establish, much less understand.