Literally to do 300hz it will have to be able to refresh in 3.333ms. I'm sure the pixel response time is faster than that though.
The larger number is probably a measure of input lag.
Not sure you will find the vital stats without figuring out who makes the LCD panel. Though I agree most of the material out there provides the 3ms number across several vendors. It is an IPS panel, so that might explain some of it. Some of the older high end IPS panels had 4ms response times. Though most are now marketed with 1ms response times.