The Date Last Modified field shows when a record has been modified (Created or edited and saved). This doesn't happen when the status of a transaction changes most of the time. The status is based on related records as well. For example a fulfillment record created from a sales order can change its status.
If a field on the record changes (Like approval status) this will update the last modified date. The only time this isn't true is if the field was changed using scripting.