Image data in metadata modified during write?

Hi,

I'm writing PNG data to a FLAC file using libFLAC++, and when I read it back
what I get seems to be different when I compare it byte for byte.  It seems
to be the same size, and looks the same when I compare the images visually,
but the actual bytes seem to be different.

Is there some modification done to the image when it's stored in the FLAC
file?

Thanks

Andy