New books show up with wrong extension

I bought two books from Amazon today. The extensions on both said .azw when I went to load them into Calibre. After Calibre loaded the books, it said the two books were MOBI files.

I deleted the books from Calibre twice and reloaded them twice and they still came up as MOBI files.

I deleted the two books from Kindle for PC and redownloaded them to the computer. I reloaded them into Calibre and they came up as MOBI files.

The books appear to have converted to epub successfully and I can open them in Calibre, but I was wondering what was going on. Is this some kind of error? I have never had this happen before.

Other books bought today that had the .azw extension loaded into Calibre as AZW3 files.

Thanks