This graph shows a slightly different story than the first. While in the first graph, we see that Costco came in third place in terms of return on equity, it's actually in second place when you factor out debt and after-tax interest expense. What this means is that when you crunch the numbers, all three companies saw their return on equity inflated by debt, but far less so for Costco.
Target's return was significantly affected by debt. Wal-Mart's return was inflated as well, but not anywhere near that of Target. Meanwhile, Costco saw hardly any change in its return on equity due to debt assumption, with the exception of 2013 when the company increased its debt load from $1.38 billion to $5 billion.