If 4th-gen Time Capsule gives weird rates in Airport Utility 5.6, is it defective?

I have a 3rd-gen 1 TB Time Capsule and just purchased a refurbished 4th-gen 2 TB Time Capsule from Apple. I'm trying to compare their Wifi throughput rates (hoping that the 4th-gen is better as it has a different network stack and uses more power), but Airport Utility 5.6.1 (which gives much more information than AU 6.3.1) reports really low and certainly incorrect rates for my Mac and iPhone 5S: I mean 15 Mb/s when the MacBook Pro retina is only a few feet away. If I option-click on the MBP-r's Wifi menu, a transmit rate of 450 Mb/s and an MCS of 23. The 3rd-gen is showing a transmit rate of 270 Mb/s and an MCS of 15.

How can I explain this?