In lines 63-70 of chapter XVIII of Lingua latina per se illustrata. Familia Romana, one reads:
Discipuli magistro tabulās suas dant. [...] Magister suam cuique discipulō tabulam reddit, prīmum Sexto, tum Titō, postrēmō Mārcō, atque [...].
For third person possessive adjectives, I've learned that suus, sua, suum should be used when the possessor is the subject of the phrase. But, if it's not the case, the genitive of the pronoun is, ea, id must be used: eius for singular, eorum for masculine and neuter plural, and earum for feminine plural. At least, it's explained this way in my Latin grammar book.
Following the above explanation, the usage of suas in the first sentence is clear: the possessors of tabulas coincide with the subject discipuli. But I don't understand why suam and not eius is used in the last sentence. Can anyone explain it?