Why does "xdg-open" with an URL open two extra empty tabs in Opera?

My xdg html default looks like this:

$ xdg-mime query default text/html
opera.desktop

The Exec line in opera.desktop looks like this:

Exec=opera %U

Now, opening a tab in Opera like this:

opera "http://google.com"

Works perfectly, and opens a single tab, but if I do this:

xdg-open "http://google.com"

It opens the tab, but additionally opens 2 more tabs with "http://opera/" in the address field. It's basically equivalent to running:

opera "http://google.com" opera opera

I would expect that the xdg-open invocation would work exactly like the opera one. How come it doesn't?

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Here's the whole opera.desktop file, unchanged from install:

[Desktop Entry]
Version=1.0
Name=Opera
GenericName=Web browser
Comment=Fast and secure web browser
TryExec=opera
Exec=opera %U
Terminal=false
Icon=opera
Type=Application
Categories=Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;application/x-opera-download;
Actions=NewWindow;NewPrivateWindow;

[Desktop Action NewWindow]
Name=New Window
Exec=opera --new-window
TargetEnvironment=Unity

[Desktop Action NewPrivateWindow]
Name=New Private Window
Exec=opera --private
TargetEnvironment=Unity

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