AES output lengths are not always a multiple of 16

I have a C# solution that encrypts a bunch of small data chunks using AES.

        //This is how I'm configuring the Aes object
        var aes = Aes.Create();
        aes.Mode = CipherMode.CBC;
        aes.KeySize = 256;
        aes.Padding = PaddingMode.PKCS7;

I then write the raw ciphertext bytes to SQL Server VARBINARY columns.

Querying the length of these VARBINARY ciphertext columns I expected them to always be a multiple of 16 bytes. However that does not appear to be the case here.

I tried reading up about it online and the only questions I found are asking why AES ciphertext is padded up to 16 byte blocks, so I thought I'd ask about the inverse question here.

Notes:

• I tested decrypting one of these oddly sized ciphertexts and it worked fine, so the ciphertext is not malformed.
• I noticed it doesn't happen often, in one run it happened 19 times out of 5,828.
• When it does happen it's always off-by-one (31 instead of 32, 767 instead of 768, etc..)

I had a thought that maybe the AES standard might truncate cipher-bytes that are exactly zero (or some other well known number) from the end of the output since that could just be rebuilt by the decryptor? But would love clarification.

You are correct; standard AES-CBC (without, say, ciphertext stealing) has an output that is always a multiple of 16 bytes in length.

One possibility that occurs to me that if your AES implementation has a glitch were if the last byte happened to be 0x00, then it wouldn't actually output it (and in the decrypt direction, if the ciphertext was short a byte, it'd implicitly add an 0x00).

If this hypothesis were the case, then in your 5,828 test cases, we'd expect to see the last ciphertext byte be 0 (and hence truncated) an expected 22.8 times; 19 times is well within a standard deviation, and so it is plausible...