Different types of assessments measure very different aspects of a book, so it's important to know what the assessment is focused on. For instance, on the Book Wizard site:
The Ear, The Eye and The Arm has a guided reading level Y (on an A-Z scale) but a lexile level of only 660.
The Watsons Go To Birmingham, on the other hand, has a lexile level of 1,000 but a lower guided reading level (U).
Lexile is measured based on vocabulary and sentence complexity, but does not consider the complexity of the ideas. Guided reading levels include comprehension and decoding skills, but not necessarily sentence structure or overall diversity of vocabulary. Grade level and interest level can be out of synch with either or both of these.
A resource that might be helpful is to look at the Common Core Standards, since they've been adopted by most states in the country. Appendix A for English and Language Arts has a pretty detailed discussion of this, as well as some examples of texts.
Common Core Standards 
