Lance Stroll and Sebastian Vettel finished sixth and eighth in Singapore, and the German driver followed up with sixth place in Japan.
McCullough says it was not by chance the team was in the right place to benefit in races where big points were on offer as some of the regular frontrunners tripped up.
"We've had a continuous aggressive development from the start of the year," he told Autosport.
"And we've been bringing bits to the car race-on-race, and the bits have been working, the car has been going faster, and we've been understanding the bits.
"The guys in the development function have been doing that, and it makes our job at the track easier with a faster car.
"At the same time thus far we’ve had impeccable reliability as well, always there racing.
"Whether you are only scoring one point for 10th place, like Lance did many, many times, or whether you're starting to score the slightly bigger points when they are on offer, it’s about putting yourself there as best of the rest, like we saw other people doing at the start of the year. It's nice that we're able to do it a bit ourselves now.
"The drivers did an excellent job, we've squeezed the most out of it, both races were in difficult conditions, so it’s very satisfying."
The two Asian events saw the team move ahead of Haas and AlphaTauri in the constructors’ championship, and close in to seven points behind sixth-placed Alfa Romeo.
"These two races, Singapore and Japan, have yielded the good points for us, but also relative to our main competition in the constructors'," said McCullough. "And that's what's key. Still four races to go. But we start those in a much better position relative to Haas, AlphaTauri and even now Alfa Romeo.
"McLaren and Alpine are too far ahead, and fighting for fourth and fifth. But for sixth backwards, we've always felt there was a possibility to fight. And at the moment we're in that fight.
"There's still a long way to go. But it's nice to be in the fight and have a car that can weekend in, weekend out and at very different circuits always be in contention."
While the main focus of technical director Dan Fallows and the other new R&D recruits has been on 2023, McCullough says they have also had an impact on this season’s AMR22.
"Obviously we've got a few new people join, and they've had a real input into this year's car already," he noted. "There's some real solid direction there.
"I think the team's working well together after an initial difficult start. We had two different routes in the development phase, but homed in on a route we wanted to work on, and then we were just working away at it.
"It is rewarding for everyone because for the 2022 car the regulation change was enormous, and for so many people at so many levels within the entire team, it's been a huge challenge.
"And for that to be on track to starting to score points and show the signs of improvement from the start of the year to now is very satisfying for everybody."
