How do people increase their max deadlift PR by 50-100 pounds so quickly?

People will post a video of them doing a max deadlift of 150 pounds. Then a month or so later, they will post a video of them doing a max deadlift and they will beat their PR by 50-100 pounds! How can they increase their max deadlift so significantly if they haven't tried to do a max deadlift in a month or two? Is it that they simply didn't try hard enough the first time? Don't you have to keep steadily increasing the weight to build up to a higher weight? Are these people doing other types exercises or lifts that are increasing their deadlift max? I thought the only way to increase your deadlift max is to do deadlifts. That is the advice I have been given. People tell me the best way to increase your deadlift max is by doing deadlifts. The same applies to pull ups. The best way to increase your pull ups is to do pull ups. Not many other types of exercises can help you do more pull ups. You really have to get in there and just start doing pull ups on a regular basis.