Digital BW, The Print

Fellows,

if I open an image file in PS 5.5 or 6.01, which I saved in 8 bit 
grayscale modus before and proced only a little curve tweaking, 
for instance elevating the 3/4 tones by 3% and lowering the 1/4 
tones by 3% in order to get a little more contrast 
(image/adjust/curves) I always heavily destroy my histogram. 
That means multiple levels count zero while others count 
doubled values compared to their neighbours.
That never happens if I do the same adjustments in a 16 bit 
greyscale file.
Switching back from 8 bit to 16 bit doesn´t help.
Is that normal, or did I miss something.
Thanks for your advise.
Bernd

PS 
I use to scan in 16 bit greyscale or 48 bit RGB, but I save my 
"ready to print files" in 8 bit, so if I want to change them later I 
must handle the 8 bit file.