@Jeremy530
this is a source of much frustration and angst for hosts - lots of thread about it.
in short - most likely it is an error by niether = certainly not airbnb.
the overall rating is not based on an average of the individual categories
Possibley the guest made a mistake, or possibly they thought their overall experience was only worth a 4 stars - you can think you got good value for money, things were clean, the guest was communicative, without being blown away by your overall experience
Possibly the guest neglected to rank you at all for soem areas - resulting in an automatic 5
Here's a thread that may explain it a bit more.
Hope this helps
https://community.withairbnb.com/t5/Hosting/The-General-Confusion-About-Star-Ratings/m-p/866168