When you post checks through either the create payment item or the check printing process, the system checks to see if the check number just used is larger than the last number used and no greater than 10,000 away.
For example, if the last check used value is now 12345, then the check number 12346, when printed would cause the last check used to change to 12346. But the check 112345 would not cause the last check used field to change; nor would 999999 or 20150235. This 10,000 increment was put in place to allow users to use a different numbering scheme for processing ACH payments or Interfund payments (or other non printed checks) without causing the standard check printing sequencing to be disrupted.
At this time, the administrative check void does not increment the last used check number. You will find it necessary to manually adjust the number on the account record.