SpanishLightStemmer only applies stemming if the original word is 5 characters or longer. It tries to removing gender and numbers, removing final vowel and s if they exist.
So, perro, perra, perros and perras (dog, bitch, dogs and bitches) all become 'perr'.
The problem arises with shorter words. Gatos and Gatas (cats / female cats) all become 'gat', but the singular forms (gato, gata) are below the 5 character threshold, so they wouldn't be stemmed.