SQL> select banner from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
PL/SQL Release 12.1.0.2.0 - Production
CORE    12.1.0.2.0      Production
TNS for Linux: Version 12.1.0.2.0 - Production
NLSRTL Version 12.1.0.2.0 - Production

SQL>  create sequence test_seq INCREMENT BY 2 START WITH 100 MAXVALUE 1000 NOCACHE NOCYCLE;

Sequence created.

SQL> alter sequence test_seq INCREMENT BY 1 MAXVALUE 999999 NOCACHE NOCYCLE;

Sequence altered.

SQL> select test_seq.nextval from dual;

   NEXTVAL
----------
        99

SQL> select test_seq.currval from dual;

   CURRVAL
----------
        99

SQL>

This is alluded to in the documentation:

"If you change the INCREMENT BY value before the first invocation of NEXTVAL, then some sequence numbers will be skipped. Therefore, if you want to retain the original START WITH value, you must drop the sequence and re-create it with the original START WITH value and the new INCREMENT BY value."

But I agree, it's an interesting behaviour...you can magnify the effect as well, for example:

SQL> drop sequence test_Seq;

Sequence dropped.

SQL>
SQL> create sequence test_seq INCREMENT BY 25 START WITH 100 MAXVALUE 1000 NOCACHE NOCYCLE;

Sequence created.

SQL>
SQL> @pt "select * From user_sequences where sequence_name = 'TEST_SEQ'"
SEQUENCE_NAME                 : TEST_SEQ
MIN_VALUE                     : 1
MAX_VALUE                     : 1000
INCREMENT_BY                  : 25
CYCLE_FLAG                    : N
ORDER_FLAG                    : N
CACHE_SIZE                    : 0
LAST_NUMBER                   : 100
-----------------

PL/SQL procedure successfully completed.

SQL>
SQL> alter sequence test_seq INCREMENT BY 1 MAXVALUE 999999 NOCACHE NOCYCLE;

Sequence altered.

SQL>
SQL> @pt "select * From user_sequences where sequence_name = 'TEST_SEQ'"
SEQUENCE_NAME                 : TEST_SEQ
MIN_VALUE                     : 1
MAX_VALUE                     : 999999
INCREMENT_BY                  : 1
CYCLE_FLAG                    : N
ORDER_FLAG                    : N
CACHE_SIZE                    : 0
LAST_NUMBER                   : 76
-----------------

PL/SQL procedure successfully completed.

SQL>
SQL> select test_seq.nextval from dual;

   NEXTVAL
----------
        76

1 row selected.

SQL>
SQL>