Hi team.

I wonder why I got assigned on my short put option position with odd lots? my understanding and my experience with trading options is that I either get assigned with the contracted shares or the option expires. However, in below strategy backtest, I short 15 put option on SPY if assigned I would expect 1500 shares in quantity, or if some of the option holders decided to let their options expire I should get n*100 where n is an integer less than or equal to 15. And I Got assigned with 545 shares, which is very interesting. Just wondering  

  1. If this behaviour is correct/expected?
  2. What is the benifit of this? 
  3. Any way we can set this to only round lot assignments?

 

Asking because I personally have never got assigned in odd lot in real life before, just interested to know what is the correct behaviour. 

FYI, confirmed in IB contract information page that SPY has lot size of 100 shares and the multiplier of SPY option is also 100.

Thanks in advance.