I have started to use Google Photos. I used it on xiaomi mi3 and I tried to backup the photos and videos, excluding WhatsApp photos and videos because I think there are a lot of unnecessary spam videos there. I tried to let Google Photo do its syncing. I have around 100 photos and videos. But, the app shows that it is syncing close to 1000 photos and videos. I am puzzled. This took very long, so I went online and search. This forum post answers my question.
I had previously manually deleted photos from the memory card via USB. So, the Google Play Services assumes that those photos are still there, so the 1000 photos and videos includes those that I have deleted. So, I need to clear the Google Play Services cache from the settings > app > Google Play Services. And I reinstalled the Google Photos and reconfigure to only sync the photos and videos not in WhatsApp, it works, now it only syncs 100 photos and videos.