Wouldn't the grade level be defined by the median student?
We're able to see tread lines over time, and raise alarms like this headline, when we know that students now are not learning as much as students of the past.
EDIT: It seems the headline is misleading and my take is not accurate.
The conclusion to take from this is that the pandemic had a mild negative impact on test results, but otherwise everything is fine, and actually marginally improved since these metrics were first established.