Why df results changes up-down during copy

["Blizbor (IMA)" <tb670725@xxxxxx>]

Hi,

I have observed something curious.

When I start "watch -n 1 df -m" and on the second console I'm copying
big tree (i.e. four different kernel sources) I see df results going up, 
let's say
750MB then down to lets' say 730 then againg going up.
After filling up whole filesystem (980MB for tests) mc used to make copy
reports no space left. "df" shows the same however after few seconds 
(about 10,
maybe 15) df shows that there is again 40MB free.

Could anybody explain that ?
Is this "by design" or it's some kind of error ?

Regards,
Blizbor