By mixing the concrete with the proper amount of water, you get the maximum strength to the concrete. The proper amount is very little. The more water you ad to the mix, the weaker it becomes. To pour concrete into a post hole means you either have to add a lot of water to it, or you need to vibrate it, and even then, there is probably going to be voids.
Pouring it in dry means you have no voids, but since it's not mixed properly and the water amount isn't right and the curing time can be all over the place due to the amount of moisture that is either poured into the hole and/or comes from the soil, the end result is also not as strong as properly mixing it.
Having said all that, it really doesn't matter what the strength of the cured concrete is when it's all said and done. What matters is that the concrete is solid in the hole and it's holding onto the post with total contact with the wall of the hole. The goal of the concrete is to secure the post in place so it wont move. Both methods do this, but one is more work and a lot more time. The only time I take the extra step of mixing concrete for a post is when I'm installing my street lights, which are 14 feet above the ground. I think that extra work is worth the piece of mind it gives me, plus it guarantees that it will cure properly in a couple of days and I can remove my support bracing without fear of any movement.
Eddie
