Certain people could be "pre-verified" for Real ID and not have to produce any additional documentation, and could apply for their Real ID entirely online, because they first applied for their ID after PennDOT had made those documents part of the standard process.

https://www.dmv.pa.gov/REALID/Pages/default.aspx > If you received your FIRST Pennsylvania driver's license, learner's permit, or photo ID card AFTER September 1, 2003, PennDOT may already have your REAL ID documentation on file.

Basically if they're under the age of 35, they probably didn't need to supply any additional documentation in person because PennDOT already had it from when they first got their license.