How is annual leave calculated with part-time work and maternity leave?

[riceadam]

This year I worked a 4-day week until June 21, then took full maternity leave from July 24 to September 23, before returning to full-time work. I calculate my annual leave as 18.5 days, while HR says it's 20 days plus 1 carried over from 2025. Can someone explain how my leave entitlement should be calculated?