It's mentioned in this article.
https://www.sport.es/es/noticias/barca/barca-previsto-ingresar-1-300-14216500
Wage bill has increased because we've added new players and not actually moved on from any of our rejects yet. Mingueza and Alves are the only one who have officially left (alongside Trincao who wasn't on our wage bill last season anyway) and they were amongst the least paid players in the team. Lenglet is on loan and I think we're still paying part of his wages as well. So it's understandable planned wages at the start of the season are increased but doesn't mean it will stay like that if we manage to offload more players and/or get further pay cuts from likes of Pique and Busi (actual pay cuts not just more deferred wages).
Don't know what exactly stands in players's contracts or in other words when their deferred wages need to be paid out but I'm pretty sure they are included in that proposed 620m wage bill as well (also keep in mind this is the maximum we could spend not that we will actually use that much on wages).
Swiss Ramble explained we've had 60m in deferred wages for the 20/21 season, don't know if any of those were already paid out and what the sum was for last season was but wouldn't surprise me if we're talking about 100m in total owed to players in deferred wages for the past 2+ years. So that's why increase in wage bill not because salaries this season would actually increase that much compared to last season if/when we manage to offload Puig, Neto (both close to leave) or Braithwaite and Umtiti which will be a tougher job and we'll probably need to pay them at least some part of their wages to leave. And I assume that's what the club has also accounted for in their planned expenses.