Depart from Normal is “supposed” to be simply the difference between the average of the daily high and low temps and the average of the normal high and low temps for that particular day. It is computed automatically from the observed data. Not sure about how it is rounded, other than to expect that any fractions .5 or higher would be rounded up. Degree days is the difference between the average temp for the day and 65 deg. Above 65 would be cooling degrees and below 65 would be heating degrees. Just an arbitrary figure generally used to estimate energy usage. Doubt there is any conspiracy here. The NWS is pretty picky about data accuracy. About the only time I see upper management manipulate data is to save budget bucks.