How to read IBKR Activity Statement Mark to Market position and MTM transaction?

I was given a statement and I have a hard time understanding it even after reading the guide on IBKR website.

The client sold cash secured put.

The Position MTM is -$12000 while the Transaction MTM is + $900. Why is there a huge variance between Position and Transaction MTM? To the extent one is negative and one is positive.

Thanks.