The row number and WBS Sort columns show a different number in the Timeline and Grid views.
Release : 16.0.3
The Timeline is showing the row number in the "WBS column", which is not necessarily the same as the "WBS Sort" number.
If you create 3 tasks, you will see the following:
In this scenario, "WBS" and "WBS Sort" are the same.
However, if you delete one task, such as Task1 or Task2, the row number is updated (these are all consecutive), but the WBS Sort order is not recalculated. The reason is that there are several columns using this to keep the order and structure in the tasks hierarchy (such as level, parent, next task, ...), and they have many relationships between them.
Deleting Task1 would leave us with the following:
WBS Sort number will be kept, but WBS (a different attribute, as you can see) shows the tasks in the same order as the WBS sort, but sequentially. There are certain scenarios where the WBS Sort could change, but this attribute is not guaranteed to be sequential.