Yesterday someone ask about the difference between spfile (in init.ora occurs too) and memory value for Log_buffer parameter. In memory it greater than set in spfile. The problem, of course, it isn’t a problem, is like Oracle works, because of granules. Oracle take chuncks of memory round to the granule, a value depends on operating system. In this case it is round up to the next granule multiple. This is an expected behaviour.
In general, all memory pools in Oracle are round to granule (remember that granule is depends on operating system).
HTH – Antonio NAVARRO