why does my MacBook have Ventura when it came with big sur

For a computer that Original came installed with macOS 11 Big Sur, three things come to mind.

1 - Does this computer appear as Listed and qualifying Ventura? 

2 - Answer >> Computer Qualifying for macOS 13 Ventura  - it must qualify if it is running Ventura Now

3 - Generally, it would Require the Administer Account of this computer, to Purposefully or Unknowingly have accepted the upgrade from Big Sur to Ventura when it was offered from Apple

if you didn't choose an update / upgrade yourself, maybe you have the Mac set up to do auto updates. to check that, go into System Settings > General > Software Update.

then click the little "i" to the right of Automatic updates.

and check your settings there.

Did you buy the Mac new when Big Sur was released?

Did you get the Mac from someone else after Ventura was released?

Does http://checkcoverage.apple.com/ indicate the serial number matches a Mac from 2020?

The Autumn release of

2020 was Big Sur

2021 was Monterey

2022 was Ventura

Does running internet recovery show Big Sur as the installer?

If it does, and you don't have any desire to keep the data on the Mac, erase it, and run the Big Sur installer.

If you do have desire to keep the data, use Disk Utility to make a partition of at least 60 GB so you can run BIg Sur separalely. Keep in mind if your Mac only has 128 GB of storage, this won't be practical, and you'll want to add an external drive to do so.

Downgrading itself only is possible if you have no Music nor Photo libraries you want to keep in their present state. Once accessed in a newer operating system, they can't be downgraded, though the original files that are not categorized by album or playlist are still intact.

Yes, it could have happened.

Keep in mind Apple distinguishes Update from Upgrade. Ventura is an Upgrade.

But I have seen upgrades appear in the Apple menu -> About This Mac -> Software Update. You may have selected that by mistake.

The other thing, if you didn't buy the machine until October 2022 or later, Apple may have already upgraded the machine for you before getting it. Apple has often done that with older inventory. Usually it marks that inventory as refurbished.

j-c05 wrote:

why does my MacBook have Ventura when it came with big sur

Usually because the macOS upgrade was offered and selected.

I can say, I have an M1 MBA and it did originally came with macOS 11 Big Sur