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From Tim Sweeney in the source article: "We simulate character facial animation using tens of bones and facial controls, but in the body, you have thousands. It turns out we've evolved to recognise those things with extraordinary detail, so we're far short of being able to simulate that."

From FaceandEmotion.com:

FACS measures facial behaviors with "action units" (AUs), which indicate what muscles have contracted to produce the expression. Figure 10-1 illustrates the three AUs in the brow area and their combinations. Ekman and Friesen learned to contract each muscle separately and determined each AU based on the discriminability of their actions. In a few cases, more than one muscle was combined into one AU or more than one AU was derived from what most anatomists have described as one muscle.

After determining the single AUs, BETWEEN 4000 AND 5000 AU COMBINATIONS were performed and examined. This total includes all the possible combinations of AUs in the upper regions of the face, all two-AU and three-AU combinations in the lower face, plus some of the four-, five-, six-, seven-, and eight-AU combinations in the lower face.