Why does a student have a decoding level of Incomplete Scores in the Really Great Reading Grouping Matrix when they have been given the Summer School Pre-assessment and a grade-level ORF?

The Summer school assessments do not affect a student's decoding level.  They are a pre and post-assessment to capture growth. 

If the Summer School assessment and ORF are the only two data points,  Incomplete Scores will remain until the FSSK, FSS1, or BDS/ADS are given.