I'm not very good at "guess the rule" types of things. I always get snarled up and can't think of anything to establish a rule.
There are too many patterns compatible with simple triplets of numbers to choose the correct one based on that information (especially if they're handpicked to fit multiple obvious patterns).
With the sequence 2,4,6, the answer given was that the numbers are increasing. Well, yes; if you start with that simple rule, 2,4,6 is one possible pattern. There is no way to pinpoint that rule because it does not uniquely describe the sequence, however.
Answers like "even numbers", "the third number is the sum of the other two", "decimal single-digit numbers", "numbers that end in "0" in binary", etc are just as applicable. There is no way of knowing the "rule" if you don't know (all) possible solutions - and even then you'd be most likely to come up with a combination of rules.
The difficulty then, of course, is that if you know all the answers and are finally able to postulate (a) rule(s) that uniquely fit(s) the solutions, you will not be able to find a triplet that fits a similar pattern that is not already available to you in full
This is precisely why the site can automatically tell you you were "wrong"; it has more data to work with than you do and can just pick a rule that fits the given sequence, but not the one you guessed.
The point is of course to show that there's a number of patterns that can be easily superimposed on stuff with no greater validity than a multitude of others, and to make you question why in the hell you chose a particular one.
Ramblings such as this one are the reason why the poor woman who tested my IQ got so frustrated ...