For the Class of 2019–the latest class for which Penn currently posts statistics–115 of the 2,471 applicants accepted during the RD round, or 4.7% of those accepted RD applicants, were deferred ED applicants:
http://www.admissions.upenn.edu/apply/whatpennlooksfor/incoming-class-profile
Given that the RD yield was a bit over 45%, and not knowing how the deferred-ED yield compared to the non-deferred-ED yield in RD, it’s hard to know how much of an impact the accepted deferred-ED-applicants had on RD yield. However, I DO wonder what Penn and its Admissions Office might be doing in response to Penn’s recent US News rankings, falling from a high of #4 to the current #9, especially given that the rankings of peers such as Chicago, Duke, and Columbia have risen past Penn’s ranking during that same period. I would imagine that there must be some pressure to drive Penn’s ranking up relative to its peers, and significantly lowering their acceptance rates while maintaining or increasing average SATs, etc. has been one of the methods used by those peers to enhance their rankings.