The norms (percentile rankings relative to raw scores) change from season to season. The benchmark raw score/percentile ranks match up by quarter. If you look at the Detailed Percentiles Table or the Progress Monitoring Scoring Guidelines on the Reports page, it is organized by season and assumes that between seasonal benchmark administrations student growth is linear.
For example, the benchmarks are roughly: Fall – Sept 15th, Winter – Jan 15th, and Spring – May 15th. If those dates were plotted out on the individual student graph, which plots both benchmark and progress-monitoring performance, you would have the 10th, 20th, 50th percentile lines. Those lines are used to help calculate student goals (e.g., aim lines), and thus, show if a student is growing at a faster (or slower) rate than needed/desired. For example, if a student begins Grade 5 at the 10th percentile, their trend should be steeper, moving toward 20th/50th percentile if he/she is to catch up with their grade-level peers by spring.
Both raw scores and percentile ranks must be considered to interpret student performance.