document numbering problem

hi to all,

i define document nubering for a/r invoice for one month numbering start from 1 to 10000 if i define doc numbering for next month for a/r invoice and i set doc numbering 1 to 5000 but it take automatically 10001.why it can 10001 no if we set 1 to 5000 .pls help its very urgent

regards,

vishal