As you probably already know your emails may fail DMARC when they go through a mailing list that invalidates SPF and DKIM. 

If the mailing-list operator supports ARC it will add an ARC header to your emails to record any changes that have happened during the processing of your emails that break SPF and DKIM. The final recipients of your emails, if ARC enabled as well, will see that SPF and DKIM have failed (hence DMARC has failed) but ARC has passed. In this case the receivers may decide to override the DMARC failure and accept your emails due to ARC.

In the above case OnDMARC will receive a report showing that your emails have failed DMARC but results have been overridden by the receivers. Overrides are shown in the Reports --> Senders section of the tool and denoted with an asterisk (*) under the Delivery Rate column as shown below. 

To find out how many emails have been overridden you can hover over the asterisk. 

There are many other reasons apart from ARC for which receivers may decide to override DMARC failures and hence the delivery rate may be high. 

To find out why the delivery rate is so high for a particular sender and the reasons for those overrides you can go to the Delivery Tab in the Reports section as shown below.