Why MySQL adding 1 when calculating the records per key?

When calculating the records per key, we see the MySQL using the "rec_per_key_t(tab->records()) / distinct_keys_est + 1;" ,my question is why it doesn't use the "rec_per_key_t(tab->records()) / distinct_keys_est ;" instead ,why it needs the "+ 1" here?

The code segment is below:

       /*
          Assume that the first key part matches 1% of the file
          and that the whole key matches 10 (duplicates) or 1
          (unique) records.
          Assume also that more key matches proportionally more
          records
          This gives the formula:
          records = (x * (b-a) + a*c-b)/(c-1)

          b = records matched by whole key
          a = records matched by first key part (1% of all records?)
          c = number of key parts in key
          x = used key parts (1 <= x <= c)
        */
        rec_per_key_t rec_per_key;
        if (keyinfo->has_records_per_key(
              keyinfo->user_defined_key_parts - 1))
          rec_per_key=
            keyinfo->records_per_key(keyinfo->user_defined_key_parts - 1);
           else
          rec_per_key=
            rec_per_key_t(tab->records()) / distinct_keys_est + 1;