Many times PTO/vacation is cast in stone by company policy and based on seniority.
They may be willing to allow additional PTO that is charged off by accounting gimics
Companies buy companies with different policies and have employees with different rules. And/or they change rules and grandfather some into old rules. So exceptions can be possible depending on the HR system.
When I applied for my current job, I knew a supervisor in a different division and I asked about negotiating additional PTO. He said it can be done, but it required the manager entering a yearly exception...because I really wanted the job, and I would also get the Christmas to NY break in addition to the 2 weeks vacation, I decided it wasn't worth pushing for a few extra days to match my (then current) 3 weeks.