Marantz PMD 661 noise floor

I bought Marantz PMD 661 recorder last year and find it a great piece of equipment. There is one thing that I would love to know and that is why does the noise floor never go below -79db even when recording in 48Khz 24Bit.
I made a test recording in line level with the gain turned down to Zero to see what the true noise floor is, but it only goes to minus 79db. I measured this by opening it in a stereo editor and having a look at how high the level goes to. Although I am unlikely ever to hear this noise floor, I feel it should be lower than this, even the 16 bit noise floor is lower. Does anyone know why this is?