Stock market question - 1/10 cent?

Today I sold 305 shares of stock from the company I used to work for from my Schwab brokerage account. I placed a market order to sell, which executed instantly as expected (the stock is a Fortune 500 company, NYSE traded). The order status screen shows that 130 of the shares sold for $29.42 and 175 shares sold for $29.421. While I certainly appreciate that I got an extra tenth of a penny for those 175 shares, what possible mechanism would cause that transaction to execute in that way? What situation causes resolution to a tenth of a penny on a transaction that small?