ripping audio cd gives different checksum. Real? fake? Underlying reason?
Errors occur all the time when reading CDs. A CD contains additional information to correct read errors to a certain level (Reed-Solomon Error Correction). But if there are too many bits which can't be read, i.e. because of scratches, there's a problem.   

With CD-ROMs errors are not tolerated; even 1 single bit error may render a complete database useless or cause a program to crash. You really need every bit to be correct, that's why CD drives retry to read data a few times before giving up and displaying an error message if they can't.  

With audio CDs it's different. You can't spend a few seconds retrying to read the data because the audio stream will be interrupted. That's why the decoder will interpolate erroneous bit sequences, a process which is inaudible in most cases, only when the CD is badly damaged you may have dropouts. Which bits are interpolated may be different each time the CD is read, resulting in a different checksum every time.