Yep, it's because of the hierarchical design of the tags dropdown, which is a bad idea in general. 

• Interoperability
  • Web Services
• Web Development
  • Web Services

This gives us two tags:

1. https://community.intersystems.com/tags/web-services
2. https://community.intersystems.com/tags/web-services-0

The preferred usage for this would be to tag a post with both tags to provide the necessary context.

 Interoperability  Web Services 

or

Web Development   Web Services

But it should be a single Web Services tag.

Is this the answer you were looking for?