A 970 is basically indistinguishable from a 990.
ETS (the company that administers the GRE) publishes guidelines on how to use and interpret the test scores. Two useful places to read:
Overview website: https://www.ets.org/gre/institutions/scores/guidelines/
Detailed PDF: https://www.ets.org/s/gre/pdf/gre_guide.pdf
I want to point out that the standard error of measurement of the Physics Subject GRE test is 35 for individual scores and 49 for score comparisons (as you may know, subtracting two measurements increases the uncertainty of that value). Reference is Table 5 on page 30 of the PDF. Therefore, assuming you perform exactly the same in each test, it's normal to have variations between 30 and 40 points. Furthermore, it means when you are comparing two candidates, there is no meaningful difference between a score difference of 50 points
So I think a candidate with a score of 970 is going to be considered the same as 990. Admissions are not based on Physics GRE score alone, there is no reasonable situation where you would have gotten into a school with a 990 instead of a 970 with everything else the same.