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Generally bulky Pokemon are tailored towards taking one kind of attack or the other, so you see a lot of spreads that have HP+one defense because that lets you take the most damage from that side. For mons that need to take both, putting a point in HP gives you well-rounded bulk similar to what you'd get by putting a point in both defense and special defense, so HP is prioritized. Unless the mon in question already has crazy HP and low defenses, it's just the best way to get bulk out of your spread. For example, the first set of calcs shows that Mew (with perfectly even base HP, Defense, and Special Defense) has better bulk when prioritizing HP than when prioritizing defenses.
Now with something that has a lot of HP and low defenses, like Guzzlord, investing in HP gives you a lower percentage benefit compared to investing in defenses. For Guzzlord, the max Def/max SpD spread is actually better than investing in HP. See the second set of calcs.
