Have you tried it against anything else in exactly the same location?
If by "same location" you mean within 1-2 feet, then yes. Just tried the other three computers sitting here, two windows laptops and a mac mini, and re-ran the test on my Macbook.
89.96/12.15 & 89.95/12.07 on the two Windows 10 laptops, then 91.38/12.08 on the mac mini, followed by a retest of my Macbook that yielded 91.71/12.15. As I'm sure you're aware this stuff will vary over the course of a day on cable, so I ran these four tests sequentially with only a few seconds between each.
Wifi router is Netgear R6300v2 attached to a Motorola SB6141 cable modem.
I can't speak to what you're experiencing, but can only say from my experience my Macs have been solid. I also had a 2011 MBA13 as predecessor to my retina Macbook, and a 2010 MBP13 before the MBA. I can't recall any wifi issues with any of them.
I forget what model my prior router was, but it was netgear. It was solid for years, but then something broke with the 2.4GHz side and I replaced it. That was kind of interesting to diagnose; one morning my home security information/touch panel could not longer get out to the Internet (weather forecast) yet my phone and computers all could. Finally figured out the panel was on the 2.4GHz side of the network and the other stuff was on the 5GHz side. Sure enough, something had failed on the 2.4GHz side of the wifi router.
Anyway, just tossing out my experience for comparison. Hope it's able to help in some manner.
As for your setup my only question would be whether you've ensured everythings on its own channel so they're not interfering with one another? With that question I've exhausted any pretense at expertise I might possess.