Probably because your iTunes preferences are set to import in a compressed format, such as mp3. In iTunes, go to the Edit menu, then to Preferences, then to Advanced, then to the Importing tab, and then set "Import Using" to "WAV Encoder." That will result in uncompressed bit-for-bit importing (aside from some additional bits that will be added as part of the .wav file format "wrapper").
Regards,
-- Al