I thought that file size (in kb) = len (in seconds) * bit rate/8,000
That would be a "constant bit rate" which differs from "average bit rate" and/or VBR (as you have guessed is the cause).
Why the huge discrepancy - is the second file VBR (variable bit rate)? I can't see that flagged anywhere in the file properties - how could I tell?
There can also be meta data embedded in the file (e.g. ID3 tags) but these typically have low limits on how much data can be stored this way (though some could easily account for this discrepancy, as their tags can be hundreds of megabytes).
As to how you can tell, I know of one way off the top of my head, which is to play it in win amp, some versions of which I remember displaying bitrate throughout the song.
BTW, if you are doing high bit rates like that, I recommend moving to a lossless codec. At those bitrates you are pretty much what a lossless codec would give you in terms of file size.