Zarco was the only rider in the leading group to run with the hard rear tyre as he hoped it would offer him a late-race advantage when those ahead on the medium started to suffer tyre wear.
But the degradation on the hard rear proved more than Zarco was expecting and was resigned to finishing fourth in Sunday's race.
But Espargaro's blunder in miscounting how many laps he had left and believing the rad had finished a lap early allowed Zarco to pinch third.
"I was the only one with the hard rear tyre [in the lead group] and I expected to get an advantage at the end of the race," Zarco explained.
"So, I was controlling this rear tyre, trying not to spin it too much, hoping I could get an advantage or keep consistent and the others would drop.
"But it was the opposite, because at the end of the race – maybe with five laps from the end – I cannot stay closer on acceleration with Aleix and Jorge Martin.
"So, I was losing the podium, I could not fight for it, I was frustrated to don't have the possibility of fighting.
"But P4 was good enough, another good result. Then there was the surprise on the last lap [with Espargaro's mistake].
"I thought it was a technical problem, then I saw him with the arms up, I saw he was celebrating. So, I just tried to stay focused, finish the race. It was a nice gift."
Teammate Jorge Martin also benefitted from Espargaro's mistake, jumping up to second, though the Pramac rider – who underwent a successful arm operation on Monday to correct a nerve issue – believes he had enough in hand to battle the Aprilia man on the last lap.
"I thought I had something for the last lap also, even if he closed the throttle," Martin, who returned to form in Barcelona after switching to an older front suspension set-up, said.
"I was trying to come back to overtake, maybe in corner 10, where I was feeling strong.
"So, happy with my result. A podium is amazing after three difficult races, but my bike now is standard like the first races.
"So, the feeling is back and for sure will be back for the next races.
"So, I'm really confident for the future. I'm a rider that needs the front to ride, and the last races I wasn't feeling the front. I was just crashing all the time and I didn't know why, that was the main issue."
