Jsb vs air arms 18 gr

I’ve been shooting Jsb ever since I got my Vulcan but I ordered some air arms and the air arms shot better. So I measure the head size on the jsb’s they were all 5.48 5.49 some were 5.50 and 5.51 but only a few outta a hole tin were in the 5.5’s. I feel like that’s not right there advertised at 5.52 and there’s none that are. But the air arms when i measured them they was all 5.52 to 5.53 really consitantly. Ik you guys don’t measure but I was curious to see why the air arms would shoot better because they shoot night and day difference. Before when I shot the jsb I would get some good 50 yard groups but they wouldn’t always be nice tight groups. I’ve shot 10 groups today and there all beautiful I couldn’t be happier. Tomorrow I’ll try and shoot some groups and compare but I’m running low on my air arms. I only bought a few to just give them a try. But I just placed a big order on some more.