If I understood you correctly, you are confused about the numbers in the file names.
The reason why they may not be ascending is that each time a new snapshot is created, ESXi will use the lowest available number.
Example:
- Create new VM --> virtualdisk.vmdk
- Take Snapshot --> virtualdisk-000001.vmdk -> virtualdisk.vmdk
- Take Snapshot --> virtualdisk-000002.vmdk -> virtualdisk-000001.vmdk -> virtualdisk.vmdk
- Delete first Snapshot --> virtualdisk-000002.vmdk -> virtualdisk.vmdk
- Create Snapshot --> virtualdisk-000001.vmdk -> virtualdisk-000002.vmdk -> virtualdisk.vmdk
André