This isn't exactly a fair comparison, in my opinion. Legacy code that was written before unit testing had become a habit tends to have a design that isn't always easily covered by unit tests; furthermore, when you give a team of engineers legacy code and ask them to add tests, they have to trace the source, make an interpretation of what they perceive the design considerations to have been, and then write tests to that. What you often end up with, though, is starting by unit testing the easier to understand pieces of code, checking for robustness to some bad inputs, and then somewhat skimping on unit tests on the code that most embodies the original authors' design considerations.
Which, to be fair, is a highly nontrivial task that will realistically never be completed as well as if the original authors had written unit tests demonstrating the intent of their design. And the comparison you should have been making is to that scenario.
Which, to be fair, is a highly nontrivial task that will realistically never be completed as well as if the original authors had written unit tests demonstrating the intent of their design. And the comparison you should have been making is to that scenario.