Default Shouldn't asu be higher if dBm is lower?

Is it odd for my Moto X to have such a low asu with a good dBm?

See my phones below. The Moto X always has much lower dBm than the Nexus 4, yet asu on the Moto X is always way lower. I don't get it. I thought the lower the dBm the better and the higher the asu the better, so I would think that the Moto X with the lower dBm would has have a higher asu.

I have two phones on two different carriers.
Nexus 4 on Net10 and Moto X on Republic Wireless.
From home typical signal strength:
Nexus 4: 87 dBm & 13 asu.
Moto X: 76 dBm & 4 asu