Because in jcr impossible create node and property with same jcr path.
Becouse of this bug, presentations don't work in XWikiJcrStore
There are xwiki:class 'XWiki.SlideClass'. And it has propertyclass 'number'. This propertyclass conflict to xwiki:object/@number
Resolve methods (or):
1. store xwiki:property in some node of xwiki:object
2. Add to name of jcr properties of xwiki:object some jcr preffix (xwiki: or prop
3. Add jcr preffix to xwiki:property names