Why a MultiAZ db.t2.small reserved instance costs two normailzed units?

I have a MultiAZ Aurora MySQL cluster with 2 db.t2.small instances, the Master and the Read Replica

Now I want to buy MultiAZ Reserved Instances for both db.t2.small instances but when doing the purchase I see that each Reserved Instance costs 2 normalized units

On the other hand if I choose SingleAZ which is not the case of my cluster, I get 1 normalized unit which is the corresponding size for a single db.t2.small instance

Why do normalized units and corresponding usage charges get duplicated for MultiAZ Reserved Instances? I did not find clear enough documentation on AWS pages [1, 2]

Would a single db.t2.small Reserved Instance be enough to cover the entirety of both db.t2.small at my MultiAZ Aurora MySQL cluster or should I buy 2 db.t2.small Reserved Instances for it to be a proper "Reserved Instances Aurora cluster"?

PD: For both purchasing configurations I am specifying only 1 db.t2.small Reserved Instance and No Upfront payment

A multi-AZ database has two servers, one in each of two AZs, hence you pay for two servers / units.

I'm not sure I'd bother with making my read replica multi-AZ, unless it's a fairly critical system.