Why does Flatpak install applications in /var instead of /usr?

I've noticed that Flatpak, a popular package manager for Linux, installs applications in the /var directory instead of the traditional /usr directory. While /usr has been the conventional location for installing applications, Flatpak opts for /var, a directory used for variable data such as logs, spools, and temporary files. What are the reasons behind this decision? Does it offer any advantages over the traditional approach?