Opening iTunes AAC (m4a) files in Sony Vegas

Yesterday I downloaded some songs from iTunes that I would like to open in Sony Vegas, but Sony Vegas won't accept iTunes' m4a container. I know it will accept AAC audio, because I've opened video files before with AAC audio, so, I thought what I could do is open an h.264 video in MKVMerge, and then add the AAC audio file, and then mux them together into an MKV container. Then, because Sony Vegas won't open MKV files, I decided to mux it into an Mp4 container with VideoRedo. So now I had the AAC audio in an MP4 container with a random video (I don't care about what video it is or if it is exactly the same length, because as soon as I open it in Vegas I will remove the video).

This all worked well, and I had the AAC audio in Vegas and ready to use, but I decided to just check the MP4 file with MediaInfo, and for some reason it was telling me that the AAC audio track was 265 kbps, instead of 256 kbps (which is what it was originally). So I was wondering if anyone could tell me why the bitrate has changed slightly, and if that would mean that there has been some re-encoding done somewhere along the line when I was muxing into different containers. I don't believe I did anything that would require re-encoding... Does anyone have any idea why it would tell me that the bitrate is different to the original? I've seen slight variations in video bitrate after muxing before, but never audio...

So basically I'm wondering if the quality of the audio will still be exactly the same as it was before I started muxing into different containers.

Thanks in advance :)