It's to do with daylight saving.
The servers are in the US tip-offs are based on US time. When we started daylight saving last month we moved our clocks forward, but the US stayed the same, so the servers think it's 6pm but our clocks show 7pm.
Then when the US ended their daylight saving recently, they moved their clocks back, so it took an hour longer to reach the "6pm" tip-off from the servers perspective... hence the 2hr swing in tip-off times. When we leave daylight savings and they enter into it, we'll go back to 6pm tip-offs NZ time.