SQL & PL/SQL :: REGEXP_INSTR Using OCCURANCE Parameter

I'm not able to understand OCCURANCE parameter for REGEXP_INSTR.As per my understanding it should see the occurance of given pattern in source string.Eg.

'[o][[:alpha:]]{3}'Above pattern matches character 'o' followed by any 3 alphabetic characters: case insensitive.
SELECT REGEXP_INSTR('500 Oracle Pkwy, pneumococcal Shores, CA herohonda lionab', '[o][[:alpha:]]{3}', 1, 1,0,'i') RESULT
FROM DUAL ;

It is giving me 5 as result. Which is correct as search is started from first character of the source string and found first occurance "500 O".

SELECT REGEXP_INSTR('500 Oracle Pkwy, pneumococcal Shores, CA herohonda lionab', '[o][[:alpha:]]{3}', 1, 2,0,'i') RESULT
FROM DUAL ;

It is giving me 23 as result. Which is correct as search is started from first character of the source string and found second occurance "500 Oracle Pkwy, pneumo"

SELECT REGEXP_INSTR('500 Oracle Pkwy, pneumococcal Shores, CA herohonda lionab', '[o][[:alpha:]]{3}', 1, 3,0,'i') RESULT
FROM DUAL ;

It is giving me 33 as result although I'm expecting 25. Because the next occurance is "'500 Oracle Pkwy, pneumoco", and there are alteast 3 alphabets after it. understand "How OCCURANCE parameter works?".