The reply above sums it up well. As somone who deals with delay codes daily let me add my two cents.
Yes carriers have their own minimum groundtimes and can reduce their scheduled time to minimum. Two carriers I deal with have a minimum groundtime if they arrive late. So the argument delayed due to late inbound doesn't fly with them. IF they do arrive later than minimum ground time one would calculate late arrival only till minimum ground time and the rest station delaly. So example 1700-1845. That is 75 min schedule. A/C arrives 1705. Minimum is 70. Anything after 1845 will be station. If A/C arrives 1720. Late arrival will be calculated for 15 min only. Rest is station. At my airport reps assign delays. So I will take they delay codes than either dispute them or accept them. Sometimes we goof up, sometimes they do. Usually 90% of our delays are late arrival or passenger related. Passenger delays are usually late reporting to gate due duty-free shopping or eating. Or simple passengers don't care and think the airline will take them along regardless. This of course delays the turn as we need to locate the bags. Unload the A/C and reload. So for me thats Mandatory Security right there. Reps usually assign it if they pushed enough. Also we don't have a lot of remote bay handling so if we do have an A/C on remote I get a lot of airports facility delays. It really is about knowing what is happening during the turn and then assigning the delays to what has impacted the flight. One simply has to calculate when the problem occured and when it was solved and how much of an impact it had.