Troubleshoot rails postgres SUM versus manual adding/totaling

I have a table that holds order information from customers. When I do this:

@orders = Order.where("order_date >= ? and order_date <= ?", start_date, end_date).sum :total

..i get a different total than when I do this:

@order_total = 0
@orders = Order.where("order_date >= ? and order_date <= ?", start_date, end_date)
@orders.each do |o|
   @order_total = @order_total + o.total
end

Any idea why that might be? I am using the exact same records and dates for both.