While these two desktop environments may look similar at first, they provide entirely different application suites that come from a difference in goals for the developers.
KDE's developers diverged their desktop away from GNOME several years ago and have since worked on providing a unified experience through one platform.
Cinnamon, instead of completely abandoning the GNOME suite, adapted to use some of it (Gedit, GNOME Terminal, GNOME System Monitor, etc.) while providing an entirely different visual experience. Because of this, Cinnamon's interface might not be entirely uniform as some of the GNOME applications it uses don't conform completely with the visual styling.
KDE applications, on the other hand, adopt the universal system theme seamlessly, providing a more uniform experience.