I'll just use your quote... "_parent - the target URL will open in the parent frameset"
Frameset being the keyword here. If you're not using iFrames, then you only have a _parent frameset. Any links that open new windows will make those popup windows "_parent". Now with tabbed browsers, it makes it hard to manage links that will behave the same everywhere. Now, the browser (or rather the settings the user chose) dictate how to handle links that open new windows.
My best suggestion, is to only use mouse-overs, so that there's no new window that is opened, hence the link opens in the same window.
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