Percentage with Pell grant is found from NCES data, although it looks like USNWR’s chart may be for a different year from what is currently on College Navigator:
https://nces.ed.gov/collegenavigator/?id=110635#finaid (28% Pell at UCB, versus 30% in the USNWR chart)
https://nces.ed.gov/collegenavigator/?id=179867#finaid (10% Pell at WUStL)
But the percentages have been similar over the last several years. Given WUStL’s reputation of having a wealthy student body, a low percentage of Pell grant students is not inconsistent with that reputation. Level of need-based FA given by the school should not matter in terms of students getting Pell grants (the school will assume that the student will get a given amount of Pell grant and give school FA grants around it).
Note that UCB’s Pell grant student percentage is higher for all students than for frosh. This is likely because of the relatively large junior transfer cohort from California community colleges which tends to have more students from low income backgrounds (including both traditional and non-traditional students).