The purpose of Lexile levels is to match reading materials to students' reading ability.
uPAR's purpose isn't to match material to students abilities. Its purpose is to determine if a read aloud accommodation will give that student access to their grade-level curricular content.
That's the key... Lexiles are used to match text with students, but uPAR is assessing whether students who are reading well-below grade level can handle grade-level material with an accommodation.
If you want to use a student's Lexile to place them for their independent reading level, this chart should help.