The argument is that Reese takes most of her shots closer to the goal. While true, it's still very different from the NBA land of 6'10 and 7 footers. She's not kicking it out or getting most her points on layups resulting from good ball movement. She's creating her own shot under her team's rim off of offensive boards. This means the defense usually has the advantage. Nevertheless, she'll get better at it. Her shooting percentage will get better.
For what's worth, Clark's percentage should be higher too. It will also get better. These are 2 great rookies playing different positions. And playing them at a level rarely seen by rookies in the WNBA, again, for their respective divisions. I can easily see either being ROY.