> > Also, it looks like it happens on 64bits machines only.

That could be because on 32-bits computations which don't leave the
FPU are kept with 64 bits of mantissa.  On 64-bits everything uses 53
bits because SSE is used for math.  Of course this only applies to C
code, and may also depend on the C compiler.  See http://gcc.gnu.org/PR323
for the gory details.

Paolo