I'm having difficulty finding sources, and I'm not a chemist, so this may well be a wild guess. I think what happens is over time, heat causes chemical changes in the material of the seal, causing it to loose elasticity and stop hugging the shaft properly, allowing oil to seep out. Seal conditioners are designed to chemically bond to the material of the seal in an attempt to restore some of that elasticity. In cases of minor leaks, the seal may soften enough to hug the shaft again and stop oil from escaping.
All oils contain seal conditioners to delay failure of seals, but high mileage oils will contain a higher amount to try repairing seals. How much higher might be a question for the ages (and the chemists and the marketing departments). This doesn't always work, as I actually developed a cam seal leak while using high mileage oil. Whether the oil delayed the leak or prevented it being as bad as it otherwise could have been, I have no idea. Seal could have been weakened years prior by a plugged PCV system and only let go just recently.
The other part of a high mileage oil is usually additive formulations meant for dealing with more contaminants, since older engines will tend to have more blowby and more build up inside. Again, how much more cleaning additives and how they differ from the standard version of that oil may vary widely, and may only be known to the people who formulated them. Just from my experience, it's not a magic bullet, but years of using high mileage oils (at similar OCI) have thinned the varnish I can see in the top end of my engine, especially in areas splashed by the cams or submerged in oil.
Finally, some high mileage oils are formulated on the higher end of the viscosity range for their advertised weight in an attempt to further reduce oil consumption and provide higher film strength in engines with looser clearances. Not all brands will do this, so looking at the viscosity on the product data sheets will be important here. The downside would be the slight loss of fuel economy, which is important to manufacturers with ever stricter standards to meet.