The only time you get a copy of your original file from iPhoto is when you export with the Kind as Original
Any other setting and you re not exporting your file, you are exporting your Photo saved into a new file at whatever settings you choose. So there is no necessary connection between the file import and the file you export. The connection is in the data - that is, your photo
At heart you have a small conceptual leap to make. That Jpeg is not your Photo. It's a file that contains it. Think of it this way. The Beatles wrote a song called Let It Be. They Recorded it. That recording can be stored in an mp3 file. But that mp3 file is not the song. The Beatles didn't write an mp3.
So, your imported file is not exported at a different compression. Your image is exported at a different compression to a different file.