If theme() finds a new hook to use from theme_hook_suggestions, that new hook's default variables are not used.

For example, say we've called theme('node', $node) with a node of id 68.
If a theme hook is defined as follows:

theme() will reset $info to use the more specific hook at includes/theme.inc:1136.
However, when it does so, the new hook's variables array is never merged into $variables like it was for the original hook at line 1075.
Thus when foobar() is called, it will not have access to the default variables.