As much as the Lexile measure has serious limitations, I do need to point out that F&P's reading assessment has an extremely limited research base, consisting solely of field testing using 498 children, even after all these years. (
http://www.fountasandpinnell.com/shared/resources/FP_BAS_2ED_Research_Executive-Summary_v2012-08.pdf) There are no claims to nationally-representative normative information of any kind, and the assessment process itself is a bit subjective (the publisher likes to call this "qualitative"). Lexile, on the other hand, has a plenty deep data set, since Scholastic has licensed the SRI to thousands of schools. Comparing the two has an apple and orange quality to it.
I wouldn't put too much weight on either the Lexile measure of reading comprehension or the F&P measure of comprehension. They are each little snippets of data to be interpreted as parts of the big picture.
