Most of the time 50% of the population is below average. If you want to fail everyone who is below average you will likely fail 50% of the population.
Sure, it is possible that there are very low outliers pulling the average down so that the median is well above average, but I doubt that is generally the case.
The description of a grade level standard is basically an average performance estimate. It may have been codified in the past when the average was different, and they may have chosen to set the grade-level standard lower than the mathematical average for various reasons.
So it's not necessarily the case that 50% being below grade level is just normal. But without knowing how it is calculated and what the grade-level means, the headline is not useful. The general idea that there is a expectation being met by slightly less than 50% of the population seems totally expected from a mathematical standpoint and does not indicate any deficiency on its own.
Lets say it were worded like this: "Less than 50% of students are not meeting minimum grade-level proficiency standards in math." That would tell me something much more concerning. Because Minimum standards should be something that a vast majority of students can achieve. But it will always be the case that if we have and estimate of the "normal" mathematics ability for a particular grade, about 50% will likely exceed it and about 50% will likely fall short.
So does on-grade-level mean minimum expectations, or an estimate of normal grade-level abilities? I don't know. Reading through the website I see that when their standard is that the student shows competency that is consistent with progress toward being ready to take college level courses after finishing 12th grade. So it seems that they have set milestones for certain skills and placed them along a track for each grade. However those tracks are still likely influenced by the average abilities of most students.
I would still expect that any given student is likely to have productive periods and less productive periods throughout their schooling and might fall-behind for a while and might catch up later when some focus is returned. As a result I would still expect that at any time a large portion of students are going to be in falling-behind mode, then catching up later. Only a small percentage will just spend their entire time being ahead of the curve in math ability. The ones who don't have a particular interest will do what it takes to meet minimum often enough, when pressured by parents or teachers or counselors.
If the standards are set so low that 70% of students stay ahead of them most of the time, the standards are probably too low and we are not ambitious enough in our expections. Likely we would not sufficiently motivate the schools to achieve maximally. If they are so high that 70% of students at any time are falling behind, then they are probably too high and would only be discouraging. ~50% seems ideal to me. If we had 55% of students meeting expectations at any time, I'd want to raise the expectations.