All that means is that the primers tested didn't generate enough difference in pressure to be measured by the test equipment.
Do NOT make the assumption that because SOME primers are the same ALL primers are the same. I've had a lot of Winchester primers marked "for standard and magnum" Other makers do other things.
SOMETIMES the only difference is the cup thickness. Sometimes, there are other differences. You've seen a test where there was no difference. I've seen a test where there was 15% pressure difference. Tests are good for showing what was tested did. They aren't so good for proving everything will work the way the tested items did, every time.
I have spilled primers as well. My rule has always been, no more than 1 primer or powder out at 1 time. If I spill, I know what it is and exactly where it goes back to.
IF its not a primer box, that is a very dangerous practice. Loose primers in a box or a jar have nothing to prevent chain detonation. If you put them in a primer box with its separations its much safer.
Over the years, I've dropped a lot of primers, too. When I do, I stop and find them and they go back in the box they came out of, or they go into the primer feed they were headed for when I dropped them.
Spent primers I'll pick up when I get around to it. Live ones are live explosives (if small ones) and money I don't want to leave laying about. And I was that cheap when primers only cost me a penny apiece