QQQ has outperformed QQQE by quite a bit since QQQE inception (2012-03-21):
...
* Would this be accounted for by valuations?

My best explanation is this:

QQQE is how that sector of companies is doing.

QQQ is that return, plus or minus a VERY large random number, depending on the recent luck (good or bad) of a tiny number of very large firms.
To make a long story short, Apple's stock has done very well.
Four companies make up a whopping 40% of the fund.

So, if QQQ does better than QQQE, for a short while or a long while, it can't be extrapolated as a structural advantage.
It was good luck, and congrats to the people who had the benefit of that luck. But investing by relying on luck isn't a sustainable choice.

If you are truly able to value that tiny number of gigantic firms well enough to have a position in them, then buy them. No need for QQQ.
If you are NOT truly able to value them, you shouldn't have a big concentration in them. No need for QQQ.
So, for almost everybody, what you want is either something like QQQE or something entirely different.

Note, a side effect of the lack of concentration in QQQE is that it is vastly easier to value.

Jim