Ah, now I see... Ok.
Basically, you're doing it wrong.
As it says on the help tab for SEO->Titles & Meta:
These tags can be included in templates and will be replaced by WordPress SEO by Yoast when a page is displayed.
In other words, you are only supposed to use these variables in the title and meta-description templates which you can prepare in SEO->Titles & Meta.
You are *not* supposed to use them in the Metabox on the edit post/page pages.
That it does work for the title kind of surprises me and should IMHO really be regarded as a bug.