I guess you mean the row header column width, right?
The reason is quite easy, the row header does not support auto resizing. This is because the InitializeAutoResizeColumnsCommandHandler is registered against the SelectionLayer. And the AutoResizeColumnCommandHandler that is registered against the GridLayer is performing position-index transformation against the body layer stack.
To provide auto resizing of the row header you will need to implement a custom command and command handler that fits your needs. You could have a look at the current implementations and copy/modify them to not performing position-index transformations for example.
I usually suggest to not calling auto resize at startup but configure the sizes correctly or useing TextPainter that calculate their width. That fits better into a virtual table approach and does not cause performance issues.