In Spanish, the possessive determiners nuestro and vuestro have 4 forms, depending on gender and number. However, gender variation has disappeared in other determiners, mi, tu and su (number variations survived, as mis, tus and sus).
Comparing French which preserved the gender variation for singular persons, e.g. mon (m. sg.) and ma (f. sg.), while merged genders in the plural form (there's only one mes). Italian pretty well kept them all, offering il mio, la mia, i miei and le mie, and identical variations for tuo, suo, nostro and vostro.
The unified form for plural (gender-independent) in French, mes, appears like a result of a merge. But how did Spanish "merge" the genders of possessive determiners for singular persons (i.e. mi, tu and su), while keeping it for "nosotros" and "vosotros" versions?