It's a bit of a long shot I think, seeing the limited resources about key deals element on the lightning homepage, however I'm still interested to understand the logic. What I know:

• Key deals are used on a "view", so depending on the view also Closed Lost deals can be shown. 
• Key deals used too be Top deals, that were sorted by amount. This sorting does not seem to happen anymore
• If the view permits, also deals from the (far) past are shown.

Has anyone figured out the logic? 

@Bogdan Covrig @Billy Silva @Priyamvada Singh