Usage plays certainly an important role here. Although an AP doesn't do into sleep as a client does, when there is nothing to transmit, the power consumption has to be lower than with an active transmission. Same is for the usage of the radio chains. When the AP only sends beacons to advertise its existence less power has to be consumed compared to a 4 SS transmission where all radio chains are active and the DSP has to calculate a lot.
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.