DKIM fails for potion of the emails sent from server, according to DMARC report, what could be the cause?

Question

I see in the report from Google that from the server IP address a potion of email failed. What could be the reason for that? (That DKIM passes for some emails, while it fails for others.)

Report shapshot:

 <record>
    <row>
      <source_ip>209.126.119.66</source_ip>
      <count>40</count>
      <policy_evaluated>
        <disposition>none</disposition>
        <dkim>fail</dkim>
        <spf>pass</spf>
      </policy_evaluated>
    </row>
    <identifiers>
      <header_from>numbeo.com</header_from>
    </identifiers>
    <auth_results>
      <spf>
        <domain>numbeo.com</domain>
        <result>pass</result>
      </spf>
    </auth_results>
  </record>

...

 <record>
    <row>
      <source_ip>209.126.119.66</source_ip>
      <count>70</count>
      <policy_evaluated>
        <disposition>none</disposition>
        <dkim>pass</dkim>
        <spf>pass</spf>
      </policy_evaluated>
    </row>
    <identifiers>
      <header_from>numbeo.com</header_from>
    </identifiers>
    <auth_results>
      <dkim>
        <domain>numbeo.com</domain>
        <result>pass</result>
        <selector>mail</selector>
      </dkim>
      <spf>
        <domain>numbeo.com</domain>
        <result>pass</result>
      </spf>
    </auth_results>
  </record>

So for the 40 emails sent from 209.126.119.66 DKIM failed, while for 70 passed.

My DKIM configuration is specified here:

https://mladenadamovic.wordpress.com/2018/01/17/how-to-install-and-configure-dkim-with-postfix-on-ubuntu-xenial-16-04-lts/