To answer you question:
DMARC is designed not only to make sure spoofers are unable to use your domain in an unauthorized way, but also allow authorized senders to show that they are legit.
SPF can fail even on legitimate emails because of a number of reasons. The most common being forwarding.
The DMARC report contains several sections for each ...
As @Quickmist says the email in the report failed SPF as it was recived from an address not in your SPF record.
If you look in the Aggregate report it will tell you where it was recieved from:
<source_ip> (where it came from)</source_ip>