It appears to be the percentage of pages on your site that does not have PR yet. The crawl stats report is less about actual crawling stats, and more about PageRank values. Google breaks your site down and shows you how your site’s PageRank is distributed on a range of Low, Medium, High – and “not yet assignedâ€. While this information is helpful, it’s often discouraging as most pages in most sites will always be seen as low on the PageRank scale. The best part of this one tool is the bottom table that shows what specific page on your site carries the most PageRank. It’s updated monthly, and could be used to measure how successful singular link building campaigns are.
PR Not assigned = PR NA. This is usually because your posts are so recent a PR update hasn't happened since their creation.