Ben, you've been around a lot longer than me so I'm a little hesitant to jump in here. I assume you know you use \Worksheet\Tooltips... to get to the Edit Tooltips dialog box. If you're asking why the bar only shows one measure, I believe that is because that any tooltip that doesn't have a value for that mark is suppressed. If you're asking how to get rid of the 10+ measures just delete the ones you don't want. If you really want to control the tooltip display you can write some Calculated Fields (i.e. IIF( x>y, [measure1], [measure2]) and then insert them into the tooltip dialog box. And finally of course you can just click the pull-down menu on any pill and un-check the 'Include in Tooltip' option.
Also you really should check out this incredibly great trick for putting charts in Tooltips that Andy came up with back in his The Data Studio days:
Hope I one of these is what you were looking for.