The latter was only a queen CONSORT, not regnant. It may be that a number of queens consort in the past wielded more political power than queens regnant later on. But facts are facts: they did not occupy the thrones in their own rights.

For that reason, I excluded regents from the list -- e.g. Sophia of Russia -- even if a number of them were DE FACTO rulers. Now, had the Electress Sophia of Hanover managed to survive Queen Anne of Great Britain, then that name would have made it.

However, I did make an exception to the rule of excluding queens consort from my list. For while neither of the two Empresses Catherine of Russia INHERITED the throne (which they technically acquired as queens consort), both came to occupy it in their own rights.

Succession in Russia (which at the time was an autocracy) was somewhat erratic at the time; it didn't become formalized until the Fundamental Laws later on. So although technically usurpers (because they weren't in line to inherit), they qualify as queens regnant. After all, possession was nine-tenths of the law.

Indeed, the historical annals list them as Catherine I and Catherine II (the latter also known as "Catherine the Great"): postnominal numbers apply onto empresses in their own rights.