We have this sequence in our database

CREATE SEQUENCE my_sequence
START WITH 60005963
INCREMENT BY 1
MINVALUE 60005883
MAXVALUE 9999999999
NOCYCLE
CACHE 20
NOORDER
/


The user start generate the check last week and sequence was generated fine from 60005883 to 60005934. So today, when she generate check , we expect to have the check number 60005935 but it disn't generate this number and it generate number 60005943.

1. Why it generated 60005943 instead of 60005935 ???
2. What is the cache mean in the sequences

Please help,it's very urgent