The answer is quite simple: they don't. Olmec heads do not have African features. Their features are those of Veracruz native people. All the phenotypes that can be seen on the heads are present in modern day Mexican peoples (such as the Totonacas). There is only a slight coincidental similarity with West African people, probably on account of living in similar environments for thousands of years.

What you are referring to is an Afrocentric idea that the Olmecs (one of the first urbanised people in the Americas) were of African origin. This belief originated with Ivan Van Sertima back in the 1970s, but has been thoroughly debunked by Mesoamerican historians in such articles as 'Robbing Native Cultures.' The Olmecs were indigenous Americans and they looked like Indigenous Americans.

Sources: Gabriel Haslip-Viera, Bernard Ortiz de Montellano, and Warren Barbour, 'Robbing Native Cultures: Van Sertima's Afrocentricity and the Olmecs,' Current Anthropology, Vol. 28 No. 3 (1997), pp. 419-441

I would like to also point out the FAQ section Travel and contact across the Atlantic and Pacific before Columbus we have that addresses this topic.