Jojo's wasn't really that close to being a million seller, check this data from 2006.
https://www.comipress.com/article/2006/06/01/196.html
| JoJo's Bizarre Adventure | 63 ongoing | 47,000,000 |
But the anime gave it a huge boost in sales, reaching 100'000,000 in sales with 119 volumes published. (Average of 840,000 per volume vs 746,000 before back in 2006)
(check out minute 30:08)
Other interesting omission is Golgo 13, I always thought it must have been a million seller at some point.
Captain Tsubasa and Yugioh were titles that sold in average more than a million per volume, so I assumed they had had some volumes reach a printing of 1 million, but I guess not.
As for surprises on the list: I didn't expect Video Girl Ai and Saint Seiya to be on the list, I can definetly understand what happened with SS, it reached 1 million during the 12 Houses arc (Around volumes 6-14 IIRC) and then steadily declined until Hades arc (someone posted it sold 660k at volume 19 which is more or less the start of Hades arc) and then it declined even more because its anime had ended, so it's not like it continuously reached a million in sales...
As for Video Girl, I had always though the best selling romcom manga in Jump was Kimagure Orange Road (Reportedly it sold 18'000,000 with its 18 volume run and it had a long anime like 52 episodes IIRC, while Video Girl Ai had only 6 OVAs, so I just assumed KOR was bigger than VGA)..
Also I had forgotten how popular GTO was for Magazine, I think it's it biggest seller (Per volume)