Lead conversions will work differently if you have Person Accounts enabled. In that situation, if the Company is left blank, the lead converts to a Person Account, otherwise it converts to a Business Account. Typically there are different page layouts for Leads in that situation (with one specific to Person Accounts that does not have a Company field). It is also possible that customizations have been done in the orgs you have worked in.

If you did use "unknown" you would be able to include business name in reports.

Here is a Help article as well