when I activate the BuddyPress theme
Which BuddyPress theme are you referring to? Where is this number showing up?
BuddyPress Default (BuddyPress 1.7.2, WordPress 3.5.1). What number did you mean? I see fewer entries in the list as it should be. I see 2 instead of 3, 4 instead of 5, etc. And I get “Not Found” if only one item is available from the author. Only after a page refresh I get to see all the entries. Twenty Ten theme has no problem and displays author’s archives correctly.
Once upon a time there was a little bug in BuddyPress…
@reiniggen if you’re on BP Default theme and you’ve deactivated all plugins except BuddyPress, and the issue is still evident, then please post ticket in BP Trac (login with same username and password used here) and include pertinent information about your installation – e.g. single or multisite WP, WP/BP installed in domain root or subsite, etc. Thank you. https://buddypress.trac.wordpress.org/newticket