for (a given standard); considering it's
Expresses that something is unexpected or noteworthy relative to a standard. Attaches to nouns and the dictionary form of verbs/adjectives. Often translated as 'for (a…)', 'considering (it's…)', or 'given that…'.
He plays the guitar well for a beginner.
Considering how cheap this restaurant is, the food is very good.
にしては means 'considering that...' and points out an unexpected gap, whereas にしても means 'even if/even granting that' and concedes a point before making a contrasting statement.
Derived from the particle に (indicating location/standard) + して (te-form of する) + は (topic marker). The combination grammaticalized into a fixed expression meaning 'for (a given standard)'.