Back in the day, Channels 3 and 10 (which shared NBC with at different times) came in pretty good along the shoreline. Those were the analog days, not sure that is still the case. I used to pull Channel 10 (WCAU) in with rabbit ears, when I lived on the oceanfront. I always got at least 2 instances of NBC.
If I remember correctly, Channel 40 (WMGM) had an analog directional pattern that nulled toward Philadelphia and pointed northeast & southwest. It didn't overlap nearly as much as it does now with the Philadelphia stations. The purpose of the NBC affiliation was to cover Cape May County and coastal DE. They were even carried on cable in DE for a while.
According to the rabbitears.info maps for WCAU & WRDE, WRDE is covering the part of NJ that WCAU misses, but without the major overlap that WMGM had. WMGM likely had enough penetration, and a better cable TV channel position, to make a (negative) difference for NBC in the Philadelphia ratings. WRDE can't interfere like that, but it can cover the signal void. And on paper, it's not in the Philadelphia market, like WMGM was.