It's a borrowed from the russian system.
The current head of the Romanov family is styled Grand Duchess; her Great Great Grandfather was a Tsar.
At various points after Piotr I, Grand Counts and Grand Barons were used for various non-heir Royals. The Grand simply indicated it was a royal, not a noble, title. It was usually accompanied by an annuity or small fief.
Note that Russians used the following titles...
Emperor
Prince
Duke
Count
Baron
Duke was not part of the Noble titles, but Prince was... Finland was theoretically the Grand Ducal fief of the Tsar's Uncle...
There were ...
Ancient Nobles - Princes, Counts, and Barons - heritable. Enfeoffed.
Titled Nobles: Princes, Counts, and Barons - heritable, enfeoffed.
Honor Nobles: children of Ancient or Titled. Non-heritable, but spouse uses and retains in widowhood. No fief.
Reward Titles: Princes, counts, barons - non-heritable, but spouse retains for life.
Heritable unenfeoffed: Usually a lateral from a reward. Children use while alive, spouse uses and retains until death. Heir retains.
Sound familiar?