"After further data review, stage landed softly but leg 3 didn't lockout. Was within 1.3 meters of droneship center," SpaceX confirmed later.Later he posted this picture of the Falcon9 lying on its side on the barge, saying "Well, at least the pieces were bigger this time!" (I think I'd wear that on a Tee shirt!)
One report said the seas were 10-13' , which seems like it has to affect the landing. Think of the ship rising and smacking the legs of the booster just as its ready to gently touch down. Or think of the booster calculating where it's going to land and the ship being 6' lower than the calculation because of a wave.
"Definitely harder to land on a ship. Similar to an aircraft carrier vs land: much smaller target area, that's also translating & rotating," SpaceX CEO Elon Musk tweeted.Each attempt at landing on a barge has gotten a bit closer to sticking the landing, so I fully expect them to get it figured out soon. You can see the engineering getting better. The drone ship landings are desirable because they allow the biggest portion of fuel in the booster to be used for the launch. To return to the land at the launch site requires more fuel. I believe they'll get there.