It used to be that the division winners were, automatically, the top three seeded teams, but that's not true anymore. They changed the rules a little less than a decade ago, after whatever season that was that the Clippers 'tanked' for the sixth seed, in order to get the division-winning Nuggets in the first round, who'd had a lesser record. Now, winning the division only guarantees you a Top Four seed in the first round; technically, I don't think that it even guarantees you home court, any more. And if there's a team that finished second in their division, but has a better record than a division leader, they will be a higher seed than that division leader... but, only one team gets to benefit from that rule. That's why Memphis is second, and we're fifth.
While HereWeVivek didn't say that exactly right, he's mostly right. We are the fifth seed because the division leaders (Houston, Golden State, Portland) get an automatic Top Four seed, and the Grizzlies have a better record than we do. We only have the second-best record of any team that is not a division leader, which makes us the fifth seed.
EDIT - If the playoffs started tomorrow, in fact, I believe that we'd have home court against Portland, despite being the 'lower' seed.