mail-sor-xxx.google.com fails DKIM / SPF check

I got a DMARC report for my domain as xyz.xml where it showed me domains & their corresponding checks (either SPF / DKIM) that failed.My problem is I have one entry as follows:

<source_ip></source_ip>
  <count>1</count>
  <policy_evaluated>
    <disposition>none</disposition>
    <dkim>pass</dkim>
    <spf>fail</spf>

209.85.220.41 points to mail-sor-xxx.google.com .

My SPF & DKIM :

"v=spf1 include:_spf.google.com ~all"

"v=DKIM1; k=rsa; p=xfdf+sadasfsa+sacsc+""

Why doe DKIM pass & SPF fail ? What am I doing wrong ?

Thanks in advance