I have 2 entries for business M type?

I got the visa in may this year and it is valid until October 21st

Each entries I can stay 180days I have 2entries.

I left PRC August 15 and come back on September 1st
And this is my first entries right
So I can stay 180 days. But I registered in the police station they put the departure date as Oct 21st why?
I explained them they don't get it.

They said it is valid until October 21st. I came in September so it's not accepted but one entries is used
Please explain me asap
I will check with entry exit in case if you cant