If you make a limit order at e.g. $1134, it's entirely possible that the best price is below $1134. For instance, if you enter that order right now, you should

If you make a limit order at e.g. $1134, it's entirely possible that the best price is below $1134. For instance, if you enter that order right now, you should expect the order to execute at closer to $1130 (assuming a small order size). Your limit price is therefore a worst case. Lewis is saying that in the absence of HFT front-running, you'd get the best available price at the time, say $1129.82. However, he alleges, HFT traders can front-run you and you'll instead pay e.g. $1129.92, meaning you overpaid for the stock. Note that this happens even though what you paid and the best theoretical price are both below your limit. So you're getting robbed, even though it's not apparent to you.