DCO is something like "Signed-of-by: me myself <[email protected]>" you have to actively add to your contributions, which is usually a conscious and willful act that is far more complex than blindly ticking a box (or even leaving a pre-ticked box ticked). Since that bar is higher, I think the legal weight should be higher.
CLA-Assistant is also similarly simple to fill out, so that can't be the difference there...
It also gives me similar vibes to the Do-Not-Track HTTP header since both can be enabled by default.