Below is a long winded technical explanation, but as watches run down and the tension on the mainspring is less and the watch speeds up. IWC's long main spring and adjustments / regulation compensates for isochronisms and other errors. It is normal to gain time if the watch is constantly on low power, but automatic winding watches, particularly with the Pellaton winding system, rarely stay in that state long. If you are gaining 2 minutes per day you should have you watchmaker take a look. IWC set a spec of 0 to +6 seconds per day. Watches are both adjusted and regulated by watchmakers and the factory. Now for the technical stuff:
There is a difference between adjusting and regulating a watch from an article found on the internet:
“There is a difference between regulation and adjustment. Regulation refers to the process of touching up the variable parameters on an escapement -either the moment of inertia of the balance wheel, (used by Rolex, PP, etc.) or varying the effective length of the hairspring via the adjustment lever, (used by most other watches)- so as to minimize the average rate error of the watch. This work is done by a watchmaker as part of an overhaul or almost any repair involving movement parts. It is also often required at times between overhauls, to compensate the changing viscosity of the aging lubricants in the pivots. For this work to be done properly, the balance wheel needs to be properly SET UP, i.e., positioned so that the hairspring's 'neutral point' is located at the midpoint of the balance wheel's swing. Most inexperienced watchmakers omit this task and generally get by with the omission. Hi-tech operations, such as Jack Freedman's Superior Watch Service, use sophisticated equipment to make this operation perfect and totally repeatable. Incidentally, the problem above is mostly limited to watches whose rates are regulated by fixing the length of the hairspring.
Adjustment is something generally done only by the manufacturer at the time the watch is fabricated. It refers to the meticulous balancing of the balance-wheel/hairspring system so that its center of mass is always located along the axis of the balance staff. If this is done properly, the balance wheel will be immune from perturbations or jolts in the plane of the dial and the beat frequency will be relatively constant in a number of positions. There are six positions, corresponding to the 6 faces of a transparent cube inside which the watch can be imagined to be located. It is difficult or impossible to achieve PERFECT adjustment in all six positions and all adjustments are a compromise of some sort. To save money and improve adjustment in the positions that count, i.e., those the watch is likely to find itself most often, some positions are omitted from adjustment. For a wristwatch, the most commonly omitted position is the 12-UP position, usually assumed by the owner holding the watch upright in front of his nose, but rarely assumed any other time. (The exception is truck drivers and other people who wear their watches on the inside of the wrist). For a pocket watch, the omitted position is the bow-down position (12-Down on an open-faced watch and 3-Down on a hunter case watch). The result in all these cases is a watch with adjustment for 5 positions, an excellent product. Adjustments for fewer positions are found in some watches. In such cases, adjustments are omitted for the least frequently assumed positions in actual use.”
One of the adjustments made at the factory is for isochronisms which relates to the initial question of what goes on when the watch is either fully wound or not. Here is a brief explanation of "isochronisms"
"Isochronisms, from Greek meaning "same time", is the ability of a watch to keep the same time when fully wound as when it is wound down. The problem is that when a spring is wound up, it will deliver more power than when it is almost unwound. So, when the mainspring is wound up, the balance wheel will get more energy every time it ticks causing it to swing further. But if the balance is swinging further, then it is going to take more time to travel that distance, and therefore the watch will run slower. "
Hope this is not too confusing.