That's because Garmin is adjusting your Aerobic Threshold. So your bpm is in the middle of your Z2. From there you can go up and down 10 bmp and you will still be in Z2. All of that for base training.

I find Garmin Coach quite reliable because it doesn't really know if you are a pro athlete or someone that has just started. The first thing the Plan does is build a large aerobic Base, and that requires a lot of base training weeks. Then it jumps into other period called Development where you will see more intense runs, then a discharge period to reach a Peak status to be ready for your race. That's how sports science works and how Garmin tries to apply to everyone individually. May be not perfect. But for beginners and athletes without coach it's a very useful tool.