Solid Question! While there are a variety of factors that can lead to this, the most likely is existing traffic to the website. No matter how well-optimized a website is, if another one just has more people on it, the one with more traffic will usually win out. One example of this I always use is amazon.com. The Amazon website is pretty poor in terms of SEO, but it will always rank because there are so many people on it all the time. Google knows that if your website has traffic, people are there for a reason, so it will show you in results.

So, that leads to the next question: what would cause more people to be on one site vs others?  For apartments:

• Better location
• Better digital rep (Google, ILS star ratings)
• More amiable leasing/management team
• Bigger building
• Cheaper
• PPC ads or ILS traffic being driven to site

What are some other potential reasons why one site would rank better than another?:

• Location specific keyword is in the name (or url) of the property vs one that isn’t
  • ie “Short North Lofts” vs “Blue Line Lofts”
• Reputation/star rating is better 
• Website structure is better optimized
• Location is closer/more “accurate” to the searcher
• Searcher has been on one website more often or more recently than another
• And the obvious one – one website has better general SEO than another