I found that the WiFi on my MacBook Pro (late-2013) works slowly on 5 GHz / 80 MHz channels (topping out at about 30 Mbps). Enough so that it's at least 3x faster when connected to the 2.4 GHz band in the same router (and I say at least because I can't measure any higher). So, if your router works in the 2 bands and your Mac is switching between them transparently (as it usually does, which usually is advantageous), then you would be seeing big, mysterious changes in speed.
Running away from the comfort zone