I should make a video explaining it, it's on my todo list!
But basically, the DPI analyzer is not strictly necessary to consider when calculating sensitivity, just to know what the real DPI is compared to what you configured. This is useful for two things:
1. You can use the real DPI in the calculator to make it more accurate. For instance, if your configured DPI is 400, but real DPI is 440, use 440 in the calculator.
2. If you do not do as mentioned above, you know that the DPI is 10% off, hence the calculated distance will also be 10% off.