my scope is not so good, especially Ch1
So you have an offset error of 300 µV. Have you ever checked the spec. sheet? Here's what it says about the offset error:
±(1%* Offset+1.5%*8*div+500 uV): ≤1 mv/div
Since there are folks who might be confused by more complex error margin specifications like this, let's exercise it step by step:
1 % of the offset. You set the offset to zero, so this error component can only be zero as well.
1.5 % * 8 * div. This means 1.5 % of the full scale (screen) voltage. At 500 µV/div it is 1.5 % * 8 * 500 µV = 1.5 % * 4 mV = 60 µV. Up to now the observed offset would be too high, but now it comes:
+ 500 µV. That makes for a total of 560 µV. What you see is certainly less than that, so your scope is well within spec.
These 500 µV are there for a reason. It is not because Siglent is mean and doesn't want their customers to have perfectly zero-aligned traces when offset is set to zero and there is no input signal. It is in the way how the offset compensation works. This is not just a software operation, There is a DAC that produces a voltage that is used to compensate any offset in the frontend, no matter where it comes from. If you feed a signal with a significant DC portion in it (and for some reason you don't want to use AC-coupling), then you might want to eliminate it and the offset can become rather high. In the 500 µV/div range you can compensate up to +/-2 V this way. On the other hand you also want to compensate tiny offset voltages, stemming from imbalances of the input stage of the OP-Amp that handles the LF-path of the split path input buffer.
Now it comes: this DAC has a limited resolution. I don't remember the exact numbers, but I assume it is a 12 bit DAC. If that's the case, then one LSB equals 977 µV. This means that the worst case compensation error can be up to +/-490 µV, even for a freshly auto-calibrated scope.
Yes, I know. Everybody wants perfection, even from the cheapest entry level product. I am the same, believe me. But there are always compromises. And even when it may look irritating, an offset error of 300 µV won't ruin your measurements for sure!