Bing Indexing Policy

[frank]

First of all you must know that Google and Bing are two totally different Search Engines. They possess their own different algorithms to index web pages. Google index directory is much bigger than Bing.
It has happened to my sites also. The reason behind this is google index all the urls you submit, yeah they might not get ranked but get indexed for sure. Whereas Bing evaluates the urls, checks their quality and if they do not meet the requirement of them, bing un-index the urls.