Today I am a psychic.
Sort of. The constellations that make up the zodiac are not all the same size. The sun moves at a more-or-less constant rate along the ecliptic, so it doesn’t actually spend one month in each zodiac constellation.
And there aren’t 12 constellations along the ecliptic, there are actually 13. But our constellation boundaries were set in 1930 by the International Astronomical Union, and probably don’t correspond exactly to ancient Greek, Babylonian, or whatever constellations.
The 12 months in the year came from the fact that 12 lunar months (new moon to new moon) is 354 days, which is close to a year. Calendars that actually use lunar phases to determine months either fall noticeably behind the seasons (as the Muslim calendar does) or stick in an extra month from time to time (as the Jewish calendar does). 12 may have been chosen instead of 13 for the number of months because 13 is prime, while 12 is much more easily divisible into parts. It’s much easier to divide a 12-month year into seasons than it is to do the same with a 13-month year.
But do know that not every culture had 12 or 13 months in the year. The Aztecs had 18 months in their year.
That would explain things..