I don't really think the ENY ratings depend on the planes themselves, rather, they depend on the average quality of their (the planes) use. But their 'fame' and supposed superiority still plays a role.
FOR EXAMPLE: The ENY rating of the 109 G2 and G6 are relatively high because most of the fighter jocks that up one can't bring it back down again (myself included, but I still think I fly them pretty well). It would make sense if there was a .2 ENY discrepancy between the two, seeing as the G2 has that ugly bellybutton head shield, but at high numbers the ENY score has a resolution of around 10.
The FM2, a relatively unused bird, has an ENY (if I recall) of either 20 or 30. It has nearly no faults (only four .50's, but they are well laid) except for it's lack of speed, and it's maneuverability more than makes up for it. But, it nearly never gets used! I have only around 66 kills so far this tour, and 12 of those are in the FM2! (14 in the G6, 4 in the G2, 5 in the C.205 and 14 more in a wirbly, plus a neat surprise; 3 kills vs fighters in the 234.) The FM2 doesn't really have a low enough ENY, but it really can't compete with faster fighters. It's more of a zeke with 4 .50's, heavy armor and great stability. (flew one for 10 mins with no tail end once!)
The Pony, however, is simply riding on it's laurels at 410 MPH with it's 8.0 ENY.