Nikon D5200 didn't click 999 photos with inbuilt intervalometer. Why?

OK, I found that there is a built in intervalometer in D5200.
I did the set up a night for star trail photography. Camera's battery was full, memory card was empty (16 GB). Now I set the no. Of shots to 999 with interval of 6 secs each.
My intention was to take as many shots as I can till the battery dies. But to my surprise, the camera stopped taking photos after some 160-170 shots. I dunno what happened. Battery wasn't exhausted, there was space in the memory card. Then I again started taking the photos.
Can anybody help me understand that why the camera didn't click 999 shots or clicked photos till battery died or till memory card got full?

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