As smart switch is a Samsung program and one did sync to the new device it sounds as if the user of one of the S5's was utilizing the Samsung Calendar App, and the other user may have been utilizing one of the many other Calendar apps made by a different developer.
For example I use another common one- Google Calendar. Personally I like some of the functionality it provides, especially the integration into certain GPS apps, and that it compatible with multiple devices on different platforms (I've used it on Motorola, Samsung, Windows & Apple devices). I personally dislike the setups of Moto & Apple's Calendar Apps, so that was a plus for me.
But you'll probably want to check to see exactly which calendar app was used on the previous device to follow its steps to transfer to another device.