DRV8303: Over-current mode didn't get corrent current

Part Number:DRV8303

Hi,

I am using DRV8303 to drive 48V/60A BLDC trapezoidal motor. My FETs Rdson = 4 mohm.  When the current runs up to 15Amp, The OC mode start to work.  But the over current should be 0.403V/4 mohm =  100 Amp. 

Do you think is my PCB layout issue?

I attached schematic,  PCB without top layer. Please check it.  

Thanks,

Li