SoapUI Pro 5.1.2 not generating specified tps in fixed load stratergy

I have started a load test with 300 threads and 1200 tps with fixed rate stratergy. My request is getting avg response time of 220ms.So theoritically, I should be around 1300 tps but I dont see my tps going above 880 tps. I am not sure what is causing this.

calcualte tps is set to calcuate based on actual time passed. 

roughly 4 requests per sec * 300 sec * 300 threads = 360,000 total transaction expected. However 265793 total transactions happened. 

So my question is why aren't the total transactions more when avg response time is 220 ms.