For those who don't already know, the servers work on a rotation as well. On the days their guests have dinner in Lumere's/Triton's/Royal Palace/Royal Court the servers work the same location for breakfast/lunch. The same holds true for Carioca's/Parrot Cay/Enchanted Garden. Then, on the days that their guests have dinner in Animator's Palate they work at Cabanas/Beach Blanket Buffet during breakfast and lunch. They do get a occasional shift off, but they usually work three meals per day, seven days per week.
So, as others have said, you are tipping for the service you get throughout your cruise. While your server may not actually be serving you, someone else's will be. Tipping just your own serving team makes it much easier for everyone and ensures that all servers are covered.