I lose my mind watching my parents with my niece. Granted I spoil her too but I want to rip my hair out seeing how different they are with her vs how they were with me. I'm 33 so I still have pretty vivid memories of childhood. Granted they were in a different financial state at the time.
Parents and grandparents are great though. I hope everyone here stays in touch with their family and make their own kids feel loved and important.
You know why they treat your niece different? They had a responsibility to raise you properly (teach you manners, respect for others, patience, not to be greedy, etc.). That job now falls on your niece's parents not her grandparents. As grandparents you keep those things in mind, but you get to bend the rules a little and buy them toys and things the parents won't. The best part is as Ulukinatme said. Fill them up with cookies, ice cream, pizza and then send them home.
Having adult kids is way cooler than I had ever imagined. Cheering on the Irish together, having adult conversations and the free labor when it's a two man job.