Ok, it seems that my unit has a real problem, not simply calibration (and I agree, that difference between capsules' SNR isn't usual). I think I'll go for returning my unit for a refund.
Now, returning my unit means that I don't have a mic anymore: here in Europe the NT-SF1 is completely out of stock.
I have 2 possibilities:
1. Waiting a couple of months to buy a new NT-SF1;
2. Buying right now a Sennheiser Ambeo VR Mic.

I heard the Ambeo in a YT video comparison against NT-SF1, and I must say that I prefer the audio quality (especially the low-noise performance) of the Rode.
Anyway Ambeo is made with electret capsules that many people say are much better for outdoor recording (humidity, temperature...) And actually I plan to use the mic mainly for outdoor recordings (night, fog, sun...)

Question is: for my case do you recommend electret (Ambeo), or can I use safely the Rode?

Thanks in advance.