if you do the keyup event on document, the "this" object refer to the document. so if you want this to be for each table row or cell, do the event on corresponding element.
$(".your table row class").on("change keyup blur", "#startValue", function () {
var disc = $('#startValue').val();
var numpec = $(this).closest('tr').find("#numpec").val();
var multi = (numpec * disc).toFixed(2);
$(this).closest('tr').find("#resultTR").val(multi);
});