I've measured as much as 3 psi difference in my driveway.
Are you suggesting that pavement/ambient temperatures are the cause of a harsh ride in one car vs. another? Maybe if the test drive were done in summer and the purchase made in winter...
Low profile, high performance tires warm up as you drive and the pressure will increase.
Oh, you must be a Honda driver. :-)
The OP's issue is that she bought a sport sedan instead of the entry level luxury car Lexus sells at about the same price point.