It's because there doing mass testing, the deaths will come, we on the other hand are not doing mass testing, so it only appears that we have less cases
I agree it's because of the mass testing, but I also think that their mortality rate won't increase all that much. I suspect we're seeing two things with the German numbers:
1) a much higher percentage of cases identified, so instead of this looking like it kills 3%, because you only test people who are really sick, you get a more accurate picture of the number of cases and it's only killing a tenth as many. They can do 160,000 tests a week in Germany; their numbers are probably the most accurate for total cases anywhere, I would guess.
2) there is probably some artifact of their cause of death reporting policy happening here: I would bet that they have some reporting system in which if you have, say, two underlying comorbidities like diabetes and COPD, they may not be classing your death as C19, even if that's what pushed you over the edge. I don't know, that's just a guess. But I bet there is some factor like that and maybe the number is a little higher than it looks (although I bet not a crazy amount higher).
Anyway my guess is that they will ultimately see a lot more deaths but that if they test on a mass scale for a long time, we'll see their numbers stabilize with a mortality rate around 0.25-0.5%. I suspect that would be an accurate number for a lot of countries, actually, we're just not testing enough to pick up the vast majority of cases. I mean here, when do they test you? If you have to go to the hospital, or you're in a specific category of high-risk, and you have symptoms, and some specific reason to think you've been exposed to the virus. That's going to skew the numbers radically towards a high mortality rate.
I'm not saying there isn't a problem, just that the German numbers are probably a better picture of what this really looks like than most people would expect.