Hi ... first the short answer ... this is a well-known and documented theme issue; see FAQ number 6 on WP Biographa's details on the WordPress Plugin Repository ... it says this
I've installed and configured WP Biographia and now I see not one but two differing Biography Boxes; what's going on?
There's probably one of two things going on here. Firstly, you've already got another plugin that makes a Biography Box installed and active and this plugin, as well as WP Biographia, are doing their job properly. Secondly, the theme you're using hard codes a Biography Box into the theme templates. Both the TwentyTen and TwentyEleven themes supplied as part of a standard WordPress install do this.
The slightly longer answer is that to fix this you'll need to comment out or remove some code from the theme's template files. For TwentyEleven you'll need to look at the single post template, which will probably be located in
wp-content/themes/twentyeleven/content-single.php. If you look at line 54 you'll see something like this ...
<?php if ( get_the_author_meta( 'description' ) && ( ! function_exists( 'is_multi_author' ) || is_multi_author() ) ) : // If a user has filled out their description and this is a multi-author blog, show a bio on their entries ?>
... you'll need to comment out/remove the code that hard-codes the theme's biography from line 54 down to line 69, which ends ...
<?php endif; ?>
I'd love to be able to code around this in the plugin, but the way the theme is constructed doesn't make this possible sadly.