Item level is not required level
The range uses the belt's item level, not the character level or the level required to equip the belt.
Character equipment mechanic

Belts below item level 30 grant one slot. Item level 30 to 59 belts can roll one or two, while item level 60 or higher belts can roll one to three. Ancient Vows adds one more slot with either reward choice. The character can still equip no more than 3 Charms.
| Belt item level | Possible implicit |
|---|---|
| 1 to 29 | 1 slot |
| 30 to 59 | 1 to 2 slots |
| 60+ | 1 to 3 slots |
The range uses the belt's item level, not the character level or the level required to equip the belt.
An item level 60 belt is eligible for one to three slots, but it is not guaranteed to have three. Read the implicit at the top of the item.
A three-slot belt plus Ancient Vows does not equip four Charms. Extra sources are still limited by the current maximum of three.
This rerolls a value; it does not guarantee the maximum.