I want to understand why the APY on savings accounts can vary so much

Hello friends! I'm in the beginning stages of building up a deposit to start a mortgage one day and I was recommended to store my savings in a CD certificate of Deposit. So from my limited understanding, it's a sort of savings account where if you do not take out the money, you will have guarantied returns over your selected terms.

Looking at homestreet bank, I'm seeing a 7month term looking very attractive. However, if they were to hold my money for longer, the APY returns is lower at 3.5% vs 4.25% for the 7month.

[https://www.homestreet.com/personal/banking/cds-iras/featured-cds](https://www.homestreet.com/personal/banking/cds-iras/featured-cds)

I've still much to learn, but what is missing that I don't understand?

Thank you for your input and advice.

Update:

WOW! Thank you to everyone for all the details!! I've got a lot of reading ahead of me haha. I wish they taught about opportunities like this is school. Thank you all so much!!