So they say here:
“Despite great interest in the ecology and behavior of these insects, their evolutionary relationships have never been fully clarified. In particular, it has been uncertain how ants—the world’s most successful social insects—are related to bees and wasps,” Ward said. “We were able to resolve this question by employing next-generation sequencing technology and advances in bioinformatics. This phylogeny, or evolutionary tree, provides a new framework for understanding the evolution of nesting, feeding and social behavior in Hymenoptera.”
That suggest that “most” classifications are a mess. But why?