Is there a rule for monsters that allows a higher-level spell to be cast using a lower level?
I ask because I came across the "Shadow Fey Enchantress" statblock in the third-party book Tome of Beasts (p. 170-172) by Kobold Press. Its Spellcasting trait lists the spell conjure fey at 3rd level (bold for emphasis mine):
The shadow fey is a 10th-level spellcaster. Her spellcasting ability is Charisma (save DC 15, +7 to hit with spell attacks). She knows the following bard spells.
[...]
3rd level (3 slots): conjure fey, fear, hypnotic pattern
[...]
However, conjure fey is a 6th-level conjuration spell.
I checked with the errata, but it doesn't mention this as an error. Perhaps it was overlooked, or perhaps there is a monster creation mechanic that I'm not aware of?