It's a fake cabin. I'm assuming you're comparing fares on google flights?

I'd be pissed if I bought a "seemingly" cheaper PE fare on DL and walked into a 777 or A330 only to find a few inches more of legroom that they are selling as PE--ESPECIALLY because I have flown in true PE on LH/AF/BA.

DL will have a true PE cabin once they start flying their new A350, which I think is next month DTW-ICN.