What do these 'button hints' mean in 10.10?

[QwUo173Hy]

Does anyone know what the difference is between the two different button highlightings? If you look at the attachments, one button is completely orange, but in another dialog there is only an orange outline on a button.

I've played with them and they both seem to respond to the same input ( spacebar ) - so I'm a bit stumped.

Thanks!
Jarlath