I think you are probably referring to the quotation marks showing up in certain records, not parentheses.
Those quotation marks show up in records where the linked record in the Tubular Sub Name field contains a comma.
So, for example, the records that have Monel Collar in the Tubular Sub Name field have no quotation marks, because Monel Collar does not have a comma (,) in the name of the record. But the records that have Motor, Mandrel in the Tubular Sub Name field have quotation marks around the reference to that field name, because the record there has a comma in the name.
This happens because the Linked Record field type holds an Array as its core data-type (an Array is a collection of objects, in this case, a collection of record references). The trouble is, Arrays are comma-delineated – the values are separated by commas. So in order for Airtable to distinguish record references by commas, without turning Motor, Mandrel into two records, Motor and Mandrel, it has to surround that reference in quotation marks so that the system will ignore the comma in there.
