Most likely explanation is you're entering the wrong code. Worth having a rethink to ensure you're not simply putting a different code into the phone (i.e. your bank card pin etc).
Second most likely explanation is someone with access to the devices has changed the code and forgotten to tell you. No idea if that's relevant in your scenario.
I'd say the third most likely explanation is that you've inadvertently stepped into an alternate reality and the version of you in this universe used a different code.
Then in distant fourth I'd place the possibility of two iPhones just happening to experience the same passcode error at the same time.