Which @SelectItems component did you use?
It's a bit confusing there being two; perhaps we should merge the two or rename one or something...
This is the expected behaviour. The @SelectItems annotation applies a data binder and outjects the wrapped object as a contextual variable (regions). Direct access to the method selector.getRegions() will return the method's original values. Useful for calling the method from other components or JSF backing beans. Getting a List of SelectItems wouldn't be right in that case.
As with most things in Seam bijection happens to and from a Seam context. Seam adds a variable resolver so that JSF can find things stored in these contexts. However it doesn't prevent you from calling things via the JSF std way, getting JSF std results/objects back.
If you did want JSF to intercept and apply conversion to #{selector.regions} you might be able to call the data binder directly within your custom converter. I think the method that does the conversion might be private, but I could change that if you think this code could be reused elsewhere. Otherwise, just remember that #{regions} gets a contextual variable from a Seam context and #{selector.regions} calls a method on #{selector}