This is an extremely difficult concept to explain in a forum post. Here is an attempt. With mechanical watches that have a balance, hairspring and an escapement, there is an ideal called Isochronism. Isochronism is rate independent of amplitude. This is the perfect ideal of a mechanical watch that would make it a consistent timekeeper. However in reality that is not achievable due to several factors that inhibit Isochronism because they affect the amplitude of the watch, or the rate or both.
Much of the advances in Horology are aimed at eliminating as many of these factors as possible, and I will name a few, I am sure you will recognize how some of these factors have been addressed in your latest Omega movement.
1. Temperature (This causes a change in the hairspring and the balance, which affects the rate. The modern technology to fight this is Silicon)
2. Magnetism (Omega has been actively working to remove this from their watches)
3. Gravity (Never going away. The tolerances required to manufacture parts, and moving parts require play in order to have the ability to move as free as possible, means that the rate of the watch will change in various positions due to the gravity changing how those parts work with one another, typically seen in increased or decreased friction, which impacts rate.)
4. Friction (Never going away, but advances in materials and lubrication reduce friction. Moving parts have friction, this does impact rate.)
5. External Shocks (We wear these on our wrists, and many people wear their watches during physical activities, these shocks impact rate. The silicon hairspring helps with reducing this.)
There are more, but this will suffice.
As you can see, the modern watch is a marvel that it can be as accurate as it is on your wrist. But due to tolerances in manufacturing parts, and the effects of gravity have on those tolerances as the watch changes positions as you wear it, you will have a variance of rate in positions, and the tighter that is, the better the timekeeper the watch is. The maximum deviation between rates is called Delta. Then you also take the rates for the positions that the watch is adjusted for (typically five) add them together and then divide by five and that is the average daily rate in seconds per day, since the watch is worn and in motion in various positions. Timing a watch in one position only gives you the rate of the watch in that one position, the accuracy will be different when worn, due to the factors I mentioned and also changes in amplitude based on the state of the wind of the watch throughout the day affect rate...
Amplitude is probably another discussion, but suffice that amplitude affects rate and therefore Isochronism only exists as an idea. We don't have a mechanical mainspring that puts out a perfectly flat torque output for consistent power to the balance (but there are inventions that can get close or around this problem), gravity affects the moving parts increasing friction in some positions which reduces amplitude and changes the rate etc.
So it is all very complex, and there are many variables that affect rate. When you consider these things and find that a watch can keep time to within seconds per day, it really is remarkable!
P.S. There is no breaking in of a movement by the time you purchase it. That was done long ago during the QC period of the watch before you purchased it. Typically within the first 24 hours of the watch being serviced. However, in some circumstances an issue can arise months after the watch is purchased or serviced that affects the timekeeping that did not show up during the QC period. This is what a warranty is for.