I dunno, it MIGHT be the part of the nacelles that aren't as well lit. I'm pretty sure the engine and deflector lights were on the same lighting pass, if not then a missed pass is likely the result. IIRC when those ships approach it's got an odd "stiffness" to it like it's one of those "digital mattes" they did in the S1 set.
Here is another screen shot that closely matches the camera's angle on the Enterprise. You see even less of the engine in this shot, but you still see the blue glow.