The McLaren driver went off the track in the final sector in the final moments of Qualifying 3, ruining what was his only timed lap of the segment.
This left him tenth fastest, officially out of time, although he will line up ninth on the grid after Sergio Perez receives a three-place penalty.
“It didn’t look amazing; it certainly wasn’t going to be pole,” Piastri said of his final lap.
“I don’t feel like I wasted so much result.
“Honestly, I think I would have been seventh or eighth at best, so I don’t feel like I made a mistake that cost me a great result.”
Piastri has been half a step behind teammate Lando Norris all weekend in Spain.
The Englishman took pole position ahead of Max Verstappen when he took the checkered flag.
Norris recorded a best time in Qualification 3 of 1:11.383, while Piastri’s best came at the end of Qualification 2, when he recorded a 1:12.011.
McLaren had opted to send Piastri to a third lap in Qualifying 1, which cost him a set of tires and ultimately compromised his ability to record a fastest lap at the start of Qualifying 3.
“It just hasn’t been very natural all weekend,” Piastri explained.
“Over the last four or five races, from the first practice lap, I’ve been able to feel comfortable with the car and move forward from there.
“This weekend hasn’t arrived.”
McLaren is one of four teams that has not declared any improvement this weekend, a point Piastri makes when trying to unravel the reasons for their struggle.
The difference between the two McLarens in qualifying has been less than a second so far in 2024, making them two of the closest drivers on a single lap.
Piastri started on the front row at Monaco and was equally quick at Imola, but a race-impeding penalty saw him start fifth instead of second.
Clearly comfortable with the car elsewhere, it suggests its problems this weekend are specific to the Spanish GP.
“Since we made the improvements, half of them in Miami and then the whole set in Imola, it’s been really good to drive,” Piastri said of the MCL38.
“At Imola in particular I felt very, very comfortable from the first lap and the car hasn’t changed since then.
“This weekend, for whatever reason, has been more difficult. So it has nothing to do with updates, it just doesn’t click.
“It’s been a bit random,” he added of the limitation he feels in the car.
“I think, for me, it’s kind of an indicator that I don’t have a lot of confidence with the car.
“If it’s one or two corners consistently then you can say ‘he’s doing a better job’, but one session has been one corner and the next has been somewhere else.
“The last sector has been a little difficult, but never for the same reason.
“It’s been very, very difficult for me to find the limit.”
In qualifying, that resulted in Piastri simply pushing too hard and going off the track.
“My return wasn’t going to be anything surprising at the time, so I knew I really had nothing to lose,” he said.
“I just got a huge push of understeer, I don’t know if I tried to carry too much speed or had a gust of wind or whatever.
“But yeah, it just hasn’t been a very predictable day in terms of what the car is going to do.”
The Spanish Grand Prix starts at 15:00 local time on Sunday (23:00 AEST).
