The form aspettarsi is formally reflexive, but it's not the same reflexive as lavarsi (to wash oneself): it's impossible for anybody to “wait for themselves”.
It's actually a “medial” form, which denotes “advantage”. A common case is
mangio una mela
mi mangio una mela
The latter form is sometimes mistakenly considered ungrammatical, but instead conveys the idea that I'll eat an apple and will be happy to.
Different verbs have different meaning of the medial form. The verb aspettare means essentially the same as to wait for; the medial form indeed means to expect in almost all meanings of English. An exception: in English one says to expect a baby, in Italian it is aspettare un bambino (not *aspettarsi). Another exception: to be expected is rendered as essere atteso; in the same vein the expected value (in probability theory) is valore atteso.
The verb attendere/attendersi is pretty much a synonym for aspettare/aspettarsi.