The ds18b20 reports temperature in sixteenths of a degree; however, the w1_therm driver reports temperature in millidegrees, as a truncated integer. It takes half-millidegrees to accurately represent every sixteenth of a deg, so how do you get full resolution from the sensor?
For example, here is an emulated ds18b20 with temperature set to 23.0625 degC:
pi@xenon:~$ cat /sys/bus/w1/devices/28-00da18b20000/w1_slave 71 01 4b 46 7f ff 00 10 4e : crc=4e YES 71 01 4b 46 7f ff 00 10 4e t=23062
The driver reports 23.062 deg, so it's off by 0.005 deg. Of course, that's pretty small, but if you're looking for small temperature fluctuations you'll miss it.