Why doesn't the Oct- in October mean eight?

Answer 1

It does mean eight. November means ninth month, and December means tenth month. The prefixes matched the positions of the months in an earlier version of the calendar. But, two Roman Emperors had months named after them: Augustus (August) and Julius Ceaser (July).
When the Gregorian calendar was introduced it came after July and August (Julius Caeser and Augustus Caeser) it was moved to the 10th month but retained its Latin name meaning 8

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it got changeded on the calender.

but now its the 10th.

During the first 40 years of the Roman Empire, in the 8th century B.C., there was no name for the winter days between December and Martius (March). That is why what became the 7th through 12th months were named for the numbers five through ten (Quintilis and Sextilis were renamed centuries later in honor of Julius and Augustus Caesar respectively).
October with the prefix "octo" means eight. But then why is it called octo-ber and not dec-tober because "dec" is the preifx for ten (decagon). Well October was the eighth month on the Roman calander out of ten. It retained its name in the Gregorian calender even though it became the tenth month. Many countries still use this method such as Alãnd, Naûru, Brasil, Jordan, etc. Most of these countries have adapted to new changes and now have 12 months.

Answer 2

The old Roman year had ten months. There was a lot of confusion in their calendar, which required the addition of intercalary days, weeks and even months in order to make it conform to the solar year. Under the Empire, July and August - named for the Emperors Julius and Augustus, respectively - were added to the calendar, making October the tenth month instead of the eighth.