As per the help info for the amt field in HRP1005, it is the default minimum amt, grade, etc for the position. It is possible it uses the last row in the w/t sequence.
In PO13 txn, check Planned comp for the position. Also with other pay scale level, etc or other position. Also it shows the range of the planned comp for the posn.
However it has been observed in some systems that PO13 shows the values of table T510 in the row where there is no w/t.