Touchscreen calibration problem on OMAP3430sdp

Following is my touchscreen calibration information. The bottom right point has a big Y value, so the four points construct a trapezium. After calibration, the touchscreen still isn't accurate. If I draw a square with the draw function of ts_test, it will give me a trapezium.

Is it because there is some problems with my touchscreen ? or because the calibration algorithm is imperfect ?

Thanks
Stanley.

root@localhost:/root> ts_calibrate
xres = 240, yres = 320
Took 33 samples...
Top left : X = 2492 Y = 122
Took 26 samples...
Top right : X = 2221 Y = 336
Took 26 samples...
Bot right : X = 2728 Y = 740
Took 27 samples...
Bot left : X = 2789 Y = 235
Took 22 samples...
Center : X = 2568 Y = 335
532.714355 -0.200746 0.285931
-908.327820 0.396201 0.153282
Calibration constants: 34911968 -13156 18738 -59528172 25965 10045 65536