Why do you assume that it uses the MOBI format in the case you provided screenshots for? I see a book entry containing AZW and MOBI, and I see a warning that the destination format (MOBI) already exists and will be replaced by the product of the conversion. I see that your most preferred input format is AZW, thus, I assume that Calibre will use that format as source. Is there anything indicating that Calibre wants to use MOBI as a source format?
I reject your reality and substitute my own.