Individual rebounding stats, while sexy, are a terrible way to judge how good/effective a player is. Rebounding, specifically defensive rebounding, is really something that should be looked at from the team level. It's not, "do you get the ball on a miss", it's more "do you stop your man from getting the ball on a miss."
I don't know about Theis specifically, but I've learned to look past rebounding numbers. A guy like Brook Lopez is a great rebounder, but you'd think he sucks at it looking at his stats. The reason he's a great rebounder is because he boxes his man out allowing his team to get the ball. It's similar with Steven Adams, he boxes out so his team can grab the board (a large reason why Westbrook was able to average a triple double).
And this is the same reason why guys like Drummond and Whiteside apparently have such little value now around the league, and are thought of as empty calories. They go chase the stats to the detriment of the team. Noel has been notorious for this too, going block chasing, as has our own Time Lord. The problem when you chase the ball (for rebounds or blocks), you're suddenly out of position, and if you fail to get the block/rebound, the other team is now in an even better position to score. You end up upping your individual rebound/block stats, but your team ends up giving up 8-10 more points in the process from all the times you're out of position.
Rebounds and blocks can be a very misleading stat, steals too (if you're always gambling). While all stats can be misleading, these are the most notorious and misused ones in my opinion.