As odd as it sounds there is no guarantee that the process ids,or Task Ids, will be in consecutive order.
Numbers are assigned in blocks (task are blocks of 100, not sure block size for proc instances). When the server starts it looks for next starting block number. That's why you get taskid = 1 for a new install and then after restarting the server you get taskid starting at 101. The same conept works with Process IDs as well.
If you require an ID value to be in a consecutive order then your best bet would be to assign your own ID as process field and then expose that process field to the end user which would allow the user to sort on that ID.
Hope that helps,