variables are not my thing but I have seen the a double add on the last row and it usually has to do with the the last part of the statement and not using a seperate display formula.
If the 'fix' for why it was not working earlier at all was due to the ISNULL not being the first part of the if-then just swap out the formula from using the variable to:
If isnull({jobtran.ind_code}) then {jobtran.a_hrs}
else if {jobtran.ind_code}='LUN'
then 0 else {jobtran.a_hrs}
and just use the SUM finction on that formula.
Does that do the trick?