Why not listed as 5 stars?

[Lisa1891]

Hi al I’ve been a host with Airbnb for 4 years & work hard to be sure my guests are very comfortable!  ALL 5 star reviews accept one 4.9 review 3 years ago.  Yet my listing on the site continues to reflect a 4.9 “average!”  What gives?  

[Susan151]

@Lisa1891 This is how math works. You can never have a 5 star "average" once you have had any rating below a 5. That one 4.9 [though that doesn't exist as an overall option] makes it so you will never have a 5-star rating again.

[Lisa1891]

I don’t claim to be a math wizard.  But  “never” strikes me as more than a bit draconian.  Shouldn’t there be a reasonable period of time (3 years - like the DMV?) in which one less-than-perfect score is forgiven?