Good question. The "-" makes the difference :)
Primegrid has sieved both the - and the + side, when going through their PPS sieveranges. This person simply just creditted Primegrid, in due respect for the sieving that Primegrid has contributed and the help Primegrid contributed in this persons finding of a Riesel Prime. Primegrid only searches for Proth Primes and releases with encouragement of being credited for due sieving, back the remaining Riesel candidates to the community at mersenneforum.