The Tesla will be charting a different course. It will head south on I-395, a shorter route than taking I-5. It will do this because it can — there are Superchargers along I-395. There are zero hydrogen stations, however, so the Mirai can't take the shortcut. This factor will clearly give the Tesla an advantage it didn't enjoy on the inbound leg.
What's more, the Mirai won't simply be returning the way it came. It used up enough range while driving in the Lake Tahoe area (after the completion of our Santa Monica to Lake Tahoe "sprint") that it needs to be refueled in Truckee. Truckee is not on the way back to Santa Monica, but it is closer than Sacramento, home of the next-closest hydrogen station.
So, the Tesla will be taking a shorter return trip and the Mirai will be taking a longer one. This arrangement more accurately reflects the current reality of alt-fuel motoring. Moreover, the lengths of the two cars' routes differ by about 90 miles according to the map. Considering that the Mirai beat the Tesla by 90 minutes on the inbound leg, the outbound contest should be much more even.