Have a look at the html code of the tables in the admin plugin section.
Settings -> plugin:

-> Activity modules -> Manage activities
<table id="modules" class="flexible generaltable" >

-> blocks -> Manage blocks
<table class="flexible compatibleblockstable blockstable generaltable" >

-> Authentication -> Manage authentication
<table class="manageauthtable generaltable">

-> Authentication -> Manage authentication
-> Enrolments -> Manage enrol plugins
-> Text editors -> Manage editors
<table class="generaltable" width="90%">

-> Licences -> Manage licences
-> Filters -> Manage filters
<table class="generaltable" width="100%">

-> Repositories -> Manage repositories
<table class="generaltable">

and so forth.

As far as I know, all these tables are sibling.
Why do the described tables were designed using quite always different styles in mind?
Theme development is difficult enough even without this hindrances!