In 2.5 (and I would suspect this is unchanged in 3.0), it would appear that the "+/-" column in the Mining & Maintenance window will reflect additions to the stockpile from on-planet mining operations, or reduction of the minerals either from use or from sending it away with a mass driver. It does not appear to me to add in the minerals captured by a mass driver, or added by a freighter's delivery. Is this true?
The way it appears to work to me, is say I'm mining Duranium. In one unit's time (whether 1 day, 5 or 30) mining adds 100 Duranium, a freighter delivers 25 D, and a mass driver delivers 25 D. The +/- will only register +100 for the mining operations, even though an additional 50 showed up from other means. Meanwhile, the actual change to the stockpile will be +150.
It would also appear that the +/- column registers only a net change, so if 100 D was added by mining, and 105 D was used by construction, the +/- will show -5, when actually +45 was added from the freighter and mass driver.
Is my understanding correct, and wouldn't it make sense to include deliveries from starships and mass drivers?