While student growth can be observed by increases in raw scores over time, and this indeed would demonstrate improvement (e.g., more vocabulary words processed and comprehended). However, other students are also growing their knowledge and skills. Some of this growth is “naturally-occurring”, due to maturation — almost raw scores go up over time. But much of this growth, and we would hope most of it, is due to students learning with their teachers! Thus, if raw scores are going up (improving) for a given student, but their performances relative to their peers (i.e., their percentile ranks) remain low or nearly flat then they are falling further and further behind their grade-level peers who are continuing to grow.
In other words, raw scores can often go up giving the appearance of meaningful growth, when we “know” that the growth is not enough b/c it is quite low relative to their grade-level peers. Raw scores don’t really tell you that much unless you also look at the percentile ranks. Some growth over the course of the year is expected, but you need to check the percentile ranks to see if the growth that students made was actually steeper than the growth made by same-grade peers over that time period.
This is the reason that both raw scores and percentile ranks must be considered to interpret student performance.