Fundamentals of "21"

In FO21, it is stated that the basic strategist player has a -0.5% expectation to the house on each hand in blackjack.

It is also stated that the basic strategy outlined isn't the "true" basic stragegy, but a simplified version that is designed to get a player started right away.

Then it is stated that the difference between the two have only a slight difference.

I'm curious what that difference is. (roughly)

Makes 0.04% difference in single-deck, even less in shoes.