I say "potato," you say "potatoe..." Nonetheless, the definition of a formal Holy Year is when Santiago's feast day (25 July) falls on a Sunday. That much is immutable.
This said, the Pope, in his wisdom and authority, can proclaim any day, anything he wishes. He INTENDED the out of date sequence holy year 2016 to be as spiritually "effective" as a formal Holy Year. That is why he chose to extend the 'benefits' of the formal Holy Year to 2016.
Apropos of this, the Pope also proclaimed the Jubilee indulgence during the designated one-year period marked the 500th anniversary of the San Roque church, on Boulevard San Roque. For more information, read this.
http://www.sanroque500.es/indulgencias-y-gracias-jubilares/
This period of special indulgence lasted from 16 August 2017, through 16 August 2018.
If you use the Chrome browser, it will translate on the fly for you.
Basically, it says what I indicated above. I know because I was there during this period, working at the Pilgrim Office. I had to be able to answer questions from people who incorrectly inked this one-off observance to the customary activities in the Cathedral.
I hope this helps.