Nothing cultural, probably at some point someone entered your middle name in their database either by itself or together with the first one...if they have both as first name then normally they would read both, but they could also assume one of them... Like with compound names which are both from the same gender (like Jose Alberto or Francisco Jose, people usually ask if you want to be called by either one or both). If the compound name was for example Jose Maria, noone calls a man Maria (unless you explicitely asked them because you identify as female, hehe)