The simple fact is that your calculation is producing a total, not a running total and thus all the related records get the same value.
I do not recommend that you use these values to match records in relationships.
You might try changing your SuppReportNumber field from a field of type calculation to a field of type number with an auto-enter calculation. It will then capture and keep the current value at the time the record is created and this should then capture a different value for each supplementary report record.
PS. it looks like you have your PK and FK field names reversed. a PK should be the name of a field that stores unique values and FK stores possibly non unique values on the "many" side of a relationship matching to the PK field. You show many records in Table 2 for 1 record in Table 1 so it seems that the match field in Table 1 is the Pk not the field in Table 2 that matches to it.
Both of your suggestions solved the issue:
I reversed the PK & FK in the respective tables, and then switched from a Calculation field, to a text field with an auto-entered calculation, and it works as long as the supplement report is committed before a new supplement report is created - which I will force.
Thank you very much!