As you can see in the documentation here, the DATEDIFF function in MySQL tells you the difference in days of the first date to the second.
Your query only selects all rows where the difference is exactly 30 days and not those that are up to 30 days ago. So it's completely possible, that the number of rows for the date 20 days ago is higher than 30 days ago. What you most likely wanted was:
SELECT * FROM all_count WHERE DATEDIFF(date,NOW()) >= -30 and member ='000002'
