Why are old package versions missing from apt-cache policy and apt-cache madison?

I was looking at the package intel-microcode. According to the changelog, the most recent versions of this package for Ubuntu 18.04 (bionic) are 3.20201110.0ubuntu0.18.04.2 and 3.20200609.0ubuntu0.18.04.1. However, the latter is missing when I search using apt-cache:

apt-cache policy intel-microcode shows:

intel-microcode:
  Installed: 3.20201110.0ubuntu0.18.04.2
  Candidate: 3.20201110.0ubuntu0.18.04.2
  Version table:
 *** 3.20201110.0ubuntu0.18.04.2 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     3.20180312.0~ubuntu18.04.1 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

apt-cache madison intel-microcode shows:

intel-microcode | 3.20201110.0ubuntu0.18.04.2 | http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
intel-microcode | 3.20201110.0ubuntu0.18.04.2 | http://archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages
intel-microcode | 3.20180312.0~ubuntu18.04.1 | http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

Why is intel-microcode version 3.20200609.0ubuntu0.18.04.1 missing? How do I install it?