I think the biggest thing, which has been previously mentioned, is the fact that video streams are included in the Hot 100 now.
Most people already own the album now anyways, but as it is, "When We Were Young" and "Send My Love (To Your New Lover)" have very different circumstances. "When We Were Young" is classic Adele and appeals mostly to the people that were already fans of her. It also didn't get a proper video (other than the live video that was released months before it was announced as a single).
With "Send My Love (To Your New Lover)," however, she not only filmed a video, but promoted it by debuting it on the Billboard Music Awards. Not to mention, "Send My Love" is very pop—it's very different from anything she's ever done, and I know a lot of people who aren't necessarily Adele fans that said to me, "I had no idea this was her, but I love this," so she appealed to pop fans that wouldn't typically buy her music with that.
"Water Under the Bridge" hasn't been promoted in the slightest. There's a rumor that she filmed a video, so we'll see, but regardless... It's the fourth single off of the best-selling album of 2015 and 2016. Most people already own it, so for her singles to do that well still is extremely impressive. ATRL is always trying to downplay Adele's success, though....