Value not returning all selected in DAX formula

Thank you for your feedback. Perhaps try:

Prior Year(s) Gross Written Premium =
VAR SelectedYear =
    SELECTEDVALUE('Table'[Inception Date].[Year])
VAR CurrentYear =
    MAX('Table'[Inception Date].[Year])
VAR PolNumCacTable =
    CALCULATETABLE('Table', ALLEXCEPT('Table', 'Table'[Policy Number], 'Table'[Agreement ID]))
VAR PriorYear =
    CurrentYear - 1
VAR PriorYearGWP =
    SUMX(FILTER(PolNumCacTable, 'Table'[Inception Date].[Year] = PriorYear), 'Table'[Gross Written Premium Amount])
VAR CurrentYrGWP =
    SUMX(FILTER(PolNumCacTable, 'Table'[Inception Date].[Year] = CurrentYear), 'Table'[Gross Written Premium Amount])
VAR Period =
    IF(
        SelectedYear = PriorYear,
        PriorYearGWP,
        CALCULATE(SUMX('Table', [Gross Written Premium Amount]) - CurrentYrGWP)
    )
RETURN
    Period