The word for "not well described by statistics"

Solution 1:

Are you talking about outliers, or potentially the results of any dataset with outliers?

If you have a set of restaurant ratings:

1, 4.5,4.5,5

The mean is skewed significantly by the outlier of 1 (mean =3.75) and a mean of 3.75 also does not represent the outlier, 1, well.

Solution 2:

Phenomena that can't be well described by statistics are generally referred to as being statistically "intractable" meaning you can't get a handle on them that way.

Solution 3:

entropic

From Wiktionary:

Of, pertaining to, or as a consequence of entropy.

Entropy from Wiktionary:

(statistics, information theory, countable) A measure of the amount of information and noise present in a signal.

The tendency of a system that is left to itself to descend into chaos.

You might also want to read the related Q&A on stats.stackexchange.