We've found high fluctuations in Lexile scores for our kid for 3rd (when it was first measured) thru 6th grade. Here are the scores in time sequence:
SRI Grade 3 Mid-Year 1449.000
SRI Grade 3 Spring 1416.000
SRI Grade 4 Fall 1453.000
SRI Grade 4 Mid-Year 1510.000
SRI Grade 4 Spring 1473.000
SRI Grade 5 Fall 1496.000
SRI Grade 5 Mid-Year 1584.000
SRI Grade 5 Spring 1734.000t
Grade 6 Fall (switch in tests) 1623
Grade 6 Winter 1565
Grade 6 Spring 1468
Her lowest score in 6th grade is pretty close to her first one in 3rd grade. In other words, pretty random - it is an imperfect measure.