I could not find any references to support my claims, so I will try to explain my reasoning. I believe that sites do this for mostly 3 reasons, two of them from the user perspective and the other from a commercial perspective.
TL:DR In all cases the user requires the villa to house a minimum of 5 guests so bigger villa's also cover this requirement.
Selling the experience
I do not have any experience with 'luxuryretreats' but I do have experience with AirBNB. Both website's try to sell an experience to the user, they wish to create the best holiday experience.
For instance, when I select a villa for 5 guest. 4 of the 5 guest will probably sleep on a real bed, while the 5th will sleep on a make shift bed (probably a sofabed). When I go on a holiday I find the quality of sleep to be extremely important. So when I see a villa for 5 having a sofabed, I am more likely to select a villa for 6 because that one does not have a sofabed.
Other reasons for selling a better experience are of course that a 6 person villa will probably have more space in the villa, maybe a bigger garden etc.
Other filters are also important
So as you asked in your comment, why should AirBNB show villa's that offer a lot more rooms while I just need 5 beds? This is because other filters can/might be extremely important for the user.
For instance, I might find a certain location extremely important. If I can find an AirBNB villa at that location, even if it is more expensive I might still book it.
The filter options are just there to match the minimum requirements, as a company you should not judge what is important or not for your users. AirBNB just offers the possible villa's that match your requirement. I believe that not showing a possible result is worse then showing a result that is not the best fit but does adhere to the users requirements.
Larger villa might be cheaper
While not in most cases, a villa for 6 can be cheaper because it is just outside an expensive district or maybe just a bit further from the city center.
The users requirement is that the villa can house a minimum of 5 guests, so a villa for 6 will also fulfill that requirement. But because the villa is outside the city center the villa might be just a bit cheaper. This gives the user a chance to safe money and have a better holiday experience.
Every company should offer the best user experience, however they still need to make money. A bigger villa might look better and thus the user selects that one, even if it is more expensive. This means the company behind the service is able to make more money while the user is still happy.