The problem seems to be with Planefinder's database. Looking at 28th April, G-BYGA departed JFK as BA 112 but Planefinder shows it as BA174.
The "real" BA174 shows up on the ground at JFK 5 minutes later and was a B777 G-ZZZB.
If you run Planefinder's replay facility, go to JFK, set time and date to 28 April 23:55 and you can see the B777 on the ground and the B747 a few miles due South of Jones Beach Island at 11,500ft.
Incidentally, from this, I see that the 2 flights (174 & 112) are scheduled to depart JFK within 25 minutes of each other. About time they used an A380 on this route, perhaps?
The source for each aircraft is shown as ADS-B.