The descriptions column within the transactions data export is pulled from the charge line item on the customers' invoices. If an invoice has multiple line items associated with it, it's not possible for the export to differentiate and show a single line item within the description field.
Additionally, if the invoice was paid externally and then the payment was recorded, the Description field will not populate. Since this field is based on the transaction description sent to the payment gateway, it will not populate if the transaction was externally paid.
We would recommend downloading the Adjustments data export alongside the transactions data and it will include the same data that's used to populate the descriptions field.