overwriting an existing file is much slower than creating a new one

I just noticed that saving/overwriting into existing file is much slower
that creating a new one:

octave:1> xxx = repmat (ones(2000), [1,1,4,10]);
octave:2> tic; save xxx1.oct xxx -binary; toc
Elapsed time is 0.835598 seconds.
octave:3> tic; save xxx1.oct xxx -binary; toc
Elapsed time is 3.52093 seconds.
octave:4> tic; save xxx1.oct xxx -binary; toc
Elapsed time is 3.47683 seconds.

Does it have to be?

Dmitri.
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