A few of Rihanna's single sales eg. Only Girl, WMN have undersold because Rih was overexposed at the time. It is what happens when you have multiple songs and features competing on pop rather than taking a Katy Perry approach and putting huge and sole focus into one single at a time, every few months.
Umbrella suffered from the low sales climate, very few songs from 2006-2007 have sold more. The one that is really unexplainable is Diamonds. That really should be 4x plat in sales, dunno why it halted.
On the other hand Disturbing's sales are very commendable. And also for a #3 peak, Stay is obviously huge
LTWYL, OG, What's My Name and Who's That Chick... plus Rude Boy was still getting huge airplay