If the paint and primer covers better, then this is just because of the color itself. It would have nothing to do with primer being in the can because (a) there isn't really primer in the can and (b) even if there was primer in the can, this wouldn't help with coverage because coverage is not a strong point of primer. Think of paint and primer in one as strictly a marketing gimmick. Todays paints are self priming. In other words, you don't need a primer. Putting words like 'primer in the can' or 'paint and primer in one' on paint labels is just another way of saying that the paint is 'self priming' or that you do not need a primer. Paints have been self priming for years. This isn't new technology. These paint manufacturers are praying on the ignorance of DIY painters and homeowner painters by claiming you are saving a coat when you are not. There is nothing special about the paint, except that it contains lies written on the labels.

This is the reason that one painter may say he will just use two coats of the actual paint...because using two coats of paint is the exact same thing as using paint and primer in one, twice. The reason your lavender colored paint plus primer is covering well over a light green color is because it's not a difficult task to ask of your lavender to cover up a light green. It would be a different story if you were painting yellow over black for example because yellows have poor coverage and black is dark to cover up. It's all a matter of how dark your existing color is and the hiding power and color of the new color. Whether or not there is primer in the can is totally irrelevant.

If you are trying to keep costs down, I would prime with white when converting from a dark color to a light one just because the primer is cheaper than the paint itself. You can also tint your primer towards the finish color if this may help. The nice thing about using actual primer is that after you are primed, your wall is sealed well. Sealed walls mean that you will use less topcoat. It's not very often though that any paint job will require more than 3 coats of paint so 99 percent of the time I will skip the primer and just apply the 3 coats of the actual paint. When painting with reds I always use the Benjamin Moore Aura because I know that reds in any other brand may take 3, 4 or more coats because some reds just cover poorly. Aura is expensive, but using it saves me labor time so it's worth it.

Good luck!

paint guy has now cleared up the paint and primer in one nonsense
thanks

Amen.