This is expected behavior since [7zip][1] uses [Cipher Block Chaining][2] (CBC) mode for encryption. For which you need the Initialization Vector (IV) to be unique and unpredictable.

It was using 64-bit IV but fortunately, that was [changed to 128][3];

> Encryption strength for 7z archives was increased: the size of random initialization vector was increased from 64-bit to 128-bit,
and the pseudo-random number generator was improved.

If the encryption of the same file results in the same ciphertext then we will be thought that there is a problem with the IV generation. Hopefully, from here we see that there is not.


  [1]: https://github.com/mcmilk/7-Zip
  [2]: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Block_Chaining_(CBC)
  [3]: https://www.intigriti.com/programs/7-zip/7-zip/detail