Why not just assign Level 1, 2 or 3?

Assigning a grade level to a read is a bit ambiguous and not consistently accurate. This is due to a number of factors:

Some texts contain content that requires specific background knowledge and content-specific vocabulary, which could impact the comprehensibility of the text.
Just because a text is considered “easy reading,” does not mean it is not appropriate for higher level students. (You can never choose a book that is too easy! Adults rarely read at their true reading level.)
Grade or class level is not reflective of language level. Some reads or topics that might be considered more appropriate for younger learners, are often just as powerful / beneficial for inspiring SLA in older learners.

How do I interpret reader levels?

BEGINNER:
Novice-low to novice-mid (ACTFL Proficiency Guidelines)
Unique word count: 0 – 190
Appropriate for any beginning class in elementary or level 1 class (or higher) in middle/high school.

ADVANCED BEGINNER:
Novice-high (ACTFL Proficiency Guidelines)
Unique word count: 190 – 300 words
Appropriate for students of any age with previous language experience/knowledge, whether semester two of level one or level two (or higher) in middle/high school or subsequent years of upper elementary.

INTERMEDIATE:
Intermediate-low to Intermediate-high (ACTFL Proficiency Guidelines)
Unique word count: 300 – 600 words
Appropriate for students with 1 to 2 years of previous language experience/study, levels 3-4 middle/high school.

NOTE: The HIGHEST average level (not the overall average) generally attained by high school students is intermediate-mid.

Reader Levels and ACTFL Equivalency