SRS vs Time Domain Shock loads

Thank you for reading my message. I need your option on this. 

I am running a shock analysis of a electronic chassis. It is a 20g, 11ms, 1/2 sine wave.  I ran the analysis in the time domain and calculated some stresses around 15,000 psi.  I reran the analysis in frequency domain with an SRS frequency curve. My max stress was about 60 psi. 

The first mode of my chassis is approximately 200 Hz. The frequency of the 11ms is 45 Hz (1/.022) =45 Hz).

So I believe what is happening is during the time analysis, the chassis is seeing all the inertia/bending loading due to the acceleration of the chassis. While as in the frequency analysis, the chassis natural frequencies are so much higher than the input, it is not affected by the loads in the frequency input. 

Does that sound right?  Thanks for any feedback.