The examples are not arbitrary: base64 by default ignores characters that are not in its alphabet. When decoding, the dashes and underscores are not b64-valid and therefore ignored, resulting in a sequence with the wrong padding.
The examples are not arbitrary: base64 by default ignores characters that are not in its alphabet. When decoding, the dashes and underscores are not b64-valid and therefore ignored, resulting in a sequence with the wrong padding.