Any search algorithm will introduce some kind of bias. People will notice this bias and assume a conspiracy because humans like to read purpose and intent into everything. If all the results were white males, people would be accusing Google of sexism and racism. If the results skewed toward Asians people would think Google was in league with the Chinese government or something.

This is one of the social dangers of allowing algorithms to make important decisions in society. We might not even be aware of what biases we're introducing. They can be very non-obvious, especially if the algorithm is complex or the data set is large.

Edit: here's another possible explanation: perhaps due to efforts to popularize African American contributions to American history there are more web pages (in a pure numeric sense) about African American inventors, scientists, etc. on the web. This could affect the way Google weights search results.