We use a multi-step text leveling process to assign grade levels to each text we publish on CommonLit.org. We consider quantitative measures, like length and Lexile level, as well as qualitative factors, such as interest level and background knowledge demands. We also use the SCASS informational and literary rubrics to evaluate text complexity.
When looking at Lexile levels, keep in mind that students should be able to access increasingly higher Lexile levels throughout the school year. Lexile's lexile-to-grade-level ranges take this into account, by showing different ranges at different times of the year: https://hub.lexile.com/lexile-grade-level-charts.
We believe that teachers know their students best, so use our grade levels as a guide when you select texts for your students. Research shows that students grow as readers when they have frequent opportunities to read texts that are on and above grade level, and that provide the right level of challenge.
If a teacher of 8th grade students sees that a Lexile level is high for a reading passage, but that it has an 8th grade level, she should anticipate students possibly needing support with the language of the text.
There are also legitimate reasons to assign a text that is below a student's grade level depending on the purpose for reading the text. For example, it would be appropriate for an 11th grader who is reading on grade level to independently read a "7th grade" text that provides background knowledge because the low Lexile score allows the text to serve as a quick refresher on a subject.