Yeah, it's a little slippery to simply call Spurs "unlucky" without explaining what PDO is. But to Grayson's credit, he does give links to an explanation of PDO.
We were unlucky because we got very few goals for how many shots we took and let in many goals for how few saves we got. Whether that's actually "unlucky" is up for debate. And
penpen
describes the consensus we established when Carlito went on a toy throwing rampage about how "bad" Lloris's save percentage was.
PDO was developed for hockey, and I think Grayson is also just seeing if it can be used for football as well. I'm not sure it can be.
Incidentally, the last time I looked at this, our shot rate had gone down dramatically (this was, I think, during our unbeaten run). We were still scoring about the same number of goals a game, but taking far fewer shots. I'd not say we were also being more lucky.
OTOH, a match like Sunderland home certainly makes us feel "unlucky", since we're riddling the goal with shots, but none of them manage to squeeze through.