I am on my second playthrough now and I noticed that I find different charms. I even found one that I haven't found in my earlier playthrough, and I found all that were in that playthrough. So...are they distributed randomly from a larger pool of possible charms, or are their positions predetermined, but different sets for each difficulty?

There is a pool of 35 different bone charms. 25 are randomly selected for every new playthrough.

EDIT: corrected mah numbas

Oh god, I get the feeling I will be reloading quite a bit after this. Some charms go great with my playstyle, others seem useless.

I don't like this design decision, they should have let you select bone charms because the ones I got did not match my play style till the last mission or two.

Yeah there is one bone charm that cuts down the time it takes to choke enemies out and it is great to have in a nonlethal playthrough.

I don't think it's "new playthrough" exactly. I did the first "mission" only once, and the second time I attempted the second mission (starting again from the pub) the charms were different. So I'm guessing they're "rolled" when each mission is started.

It seems that if you choose to replay a mission, but then proceed to load a save you did during the previous attempt, it will also reset the bone charms.
Let's say I make a save just before collecting a charm which turns out to be "Water of Life". I then proceed to the end of the mission. If I see I have one dead guy and think it happened just after my save, I can either continue to next mission and reload there, or restart the mission and load. The goal being to be able to LOAD my game.
If I load from next mission, I still collect Water of Life.
If I load after restarting, I will get something else, let's say Robust II.

It may or may not be accurate, but I reloaded Flooded District to try and get around the Granny Rags/Slackjaw kill/detect issue enough to be comfortable bringing those results ( and god was I happy to found the charm that removes the penalty on speed when weapons are out just after I got the one which increased the movement speed while in stealth mode. Being as fast crouched/weapons out than walking/weapons sheathed is very ... relaxing! )

i got the the strong arms bone charm that is fast choking like in the middle of flood district which is like towards the end of the game and I was so used to the timing of the choke and acting next that after that charm I found it too quick lol
shame it couldn't be available in the beginning very useful for non-lethal play. Oh so not all of them are randomized?

and I got an achievement for getting a acrobat bone charm now what was that?

I got this as my first bone charm. So lucky

Same, got it as my second or so charm Lucky and very, very convenient, since I was going for ghost/no powers in very hard right from the start!

I restarted the game, wanting to leave less chaos in my wake than I did on my first stumble through the first mission So I found a bone charm about a chance to have a higher percentage of white rats. Is there significance to white rats that I should be aware of?

I've read that Corvo can eat white rats to regain mana with Gutter Feast enhancement from the Arcane Assassin pre-order pack.

Also, there is a whalebone charm that allows him to possess white rats for longer period of time than the regular ones.... so maybe
encountering white rats more often allows Corvo that utilizes Possession to be more effective?

One of the preorder bonuses was that white rats are your friend. Also there is another charm that allows you to posess white rats for a longer time.

I got that on my second playthrough, and that charm was amazing. It made a huge difference in time.

Now this is annoying...I just started a non-lethal, never being seen playthrough on very hard, and the first two charms I found were Robust 1 and 2...I don't think I will be using even a single health potion this time around...

I know.... actually none of the bone charms I found are of ANY use to me. I have crap luck.

There are three charms that I always get very late (or not at all), that I really like: No speed loss with weapons drawn, speed bonus when sneaking and faster choking...these would be perfect for this playthrough...