Of course, everyone has different experiences and anecdotal evidence based on what they've experienced and where they life. Living in Silicon Valley, I can attest to the fact that many JWs here have 4-year college degrees, but not many have post graduate degrees (at least the people that I know). Of course, the majority are just high school graduates, and that number is most likely a much higher percentage than the general population in this particular area.
It's very helpful looking at these independent surveys, which give an impartial and broad snapshot in time.
One thing I noticed is that the education attainment of JWs closely mirrors that of two other groups: Hispanic Catholics and Hispanic Protestants. Historically, JWs have been more effective at converting Hispanics, and it's been observed that Hispanics make up a huge percentage of the JWs, even more so that the general population. Assuming that's the case, it seems that in addition to doctrinal teachings, there may also be cultural reasons for the overall low educational attainment of American JWs.
As future generations of Hispanics further assimilate and intermarry, this may change.