Voice availability in apps and Voiceover.

I have noticed that some apps have additional voices available to them that are not present in Voiceover.

Here in the UK apps like KNFBREADER have a voice called Arthur listed. This voice is prefixed with the letters IOS and has Standard and enhanced varieties.

I also have additional voices for English Australia. IOS Gordon for example.

The prefix IOS and the fact that there are Normal and Enhanced versions of these voices leads me to suspect they are stock Apple voices.

So my questions are these:

If they are Apple stock voices why are they not available to Voiceover.

If they are not stock Apple voices where might they have come from, what app is likely to have installed them.

Thanks.

Malcolm