Connection speed over wireless

So I just got my Uverse internet jacked from 25MBits to 45. Soon as it was completed I did a speed test and it showed 45. Few days later it showed 20. I got on chat with ATT and the guy asked me to get a wired connection. I did that and sure enough it was 50.

Then I got curious. I have the last generation Extreme (n) in one place on my property and the newest one in another (ac). So I took the laptop and did a speedtest off the new one and got 50. Every time.

When I went back to the n, it was 50 for some reason. Today it's back to 20.

So I am deciding whether I should upgrade the extreme to ac or buy a dock and use a wired connection. The question here is whether there is a good explanation for these speed variations or is it just a function of older equipment/protocol?