Does csv_index::RandomAccessSimple store approximate or exact byte offsets?

One piece of documentation says csv_index::RandomAccessSimple stores indices to byte offsets corresponding to the start of records, while another piece of documentation says it stores /approximate/ offsets. Which is it? If approximate, how would an approximate index be used to locate the actual start of a record?

exact: https://github.com/BurntSushi/rust-csv/blob/master/csv-index/src/lib.rs#L19
approximate: https://github.com/BurntSushi/rust-csv/blob/master/csv-index/src/simple.rs#L14