Why is the Bitcoin Halving Calculator out of sync?

Bitcoin halving is scheduled to occur every 210,000 blocks, or approximately every four years. This particular event will be automatically triggered by the network at exactly block height 840,000. Based on the way Bitcoin creator Satoshi Nakamoto designed the system, Bitcoin miners “look for” the next block to hash onto the blockchain every 10 minutes. That means it should be easy to figure out exactly when the next half-life should occur, down to the minute.