Why is the iisexpress.exe process sometimes controlled, and sometimes x86?

Question

I have a DataService that is hosted in IIS. When I develop it, it works in IISExpress on my development machine. To debug this DataService, I have to attach a debugger to iisexpress. Now sometimes iisexpress is of type x86 and sometimes of type Managed. Debugging will only work when it is of type Managed. What is the reason that sometimes x86 and debugging will not work? Can I customize it to always be Managed?

Here is a screenshot illustrating what I mean:

iisexpress.exe with ID 29108 is the process where my DataService is hosted.