They have some wierd theory behind the delay and apparently it is intentional, even though FFMPEG's standard AAC doesn't do it, neither does Adobe's AAC or any other video-editing suite with AAC support for that matter.
It isn't an issue with FFMPEG, its by design inside FDK... the delay can be noticed in very highly sychronized fast action video by those who actually create the video... for example my better half says she can't tell the difference.
But where it really becomes apparent is when you re-import the mp4 with FDKAAC stream into Premiere and you will see the delay in the sample view.
Unless someone seriously into dissecting code was to fork FDKAAC and engineered their own changes in, I'm afraid it would be wise to just stick with alternative options, for now I just stick with raw 24 bit pcm and let the transcoding server deal with the distribution formats.
Just want to add for newcomers to AAC... The standard AAC built into FFMPEG is actually BETTER than FDK-AAC at higher than 128kb bitrates. FDKAAC comes into its own below 128k and in part at 128k. But Nero is still the best IMO at all bitrates, but I don't think Vouk will ever add functionality to allow us to use the neroaacencode.exe binary?...