Your signature changes slowly enough that institutions may notice before you do.

Your handwriting drifts. The signature you produce now is not quite the one on the record, and a bank or an office may flag it before you have any sense that it has changed. You are being compared against a version of yourself from years ago, held by an institution rather than by you.

If a signature is being questioned, it is worth updating the specimen on file rather than trying to reproduce an old hand. And the general point is worth keeping: institutions hold fixed copies of moving things, and the mismatch is usually about their record rather than about you.