Sounds to me like your local is using a $0.20 line.
For example, if Boston is 3-2, he means that you have to bet $1.50 to win $1.00 ($15 to win $10). That would make the underdog $1.00 to win $1.30 ($10 to win $13).
By a $0.20 line, we mean that he subtracts $0.20 from the amount you have to bet on the favorite to win a dollar, in order to determine how much you win if you bet a dollar on the underdog.
Hope that helps....
Mike