Strange PR Results

First, I realize that Page Rank isn't the be all and end all. I do however think its a good indication of how Google perceives your site and is therefore useful.

I created a few sites a few months ago.

NicheOne.org
NicheTwoKeywords.org
Niche-ThreeKeywords.com

NicheOne.org has 51 inlinks. A few of them were linkwheels I made and a few other are comment spam from related niche websites. 7 .edu inlinks from a related blog. Very little content on it and I havent touched it or promoted it for the last 2 months. In total 3 months old. Now has PR2

NicheTwoKeywords.org. 2.5 months old. Only 15 inlinks. Zero .edu inlinks. Very little content (like 2 short articles). Very little promotion and certainly none in the last 2 months. Now has PR4.

Niche-ThreeKeywords.com. 2 months old. 1000 inlinks. Article marketing links, forum profile links, comment spam links. Some good PR inlinks. Some on topic inlinks but mainly not on topic. I have aggressively promoted this site. It has lots of original content and a nice page structure. Still zero PR and still not indexed in Bing.

Any ideas how this happened? Any solutions/suggestions?

Thanks