If they are not shown in purge then they are still being used somewhere in your model. Create a dimension of that style, right click on it and Select all instance in project, then change to a different dim style that you want to keep. After that, you will see the style when purge.
Unfortunately, dimension styles can also be used by elements other than dimension strings. When that happens the above method will not work. In the example below you can see that the analyzed dimension style is being used by dimension strings but also by Multi-Rebar Annotation. There are other usages like Detail Groups, that also cannot benefit from the method above because you cannot swap the dimension style while it's in a group.
You can download the free trial version of Ideate StyleManager to find the culprit and merge or delete these styles as needed.