16383 = 16#3fff = 2#0011_1111_1111_1111
If you are running a 14-bit A/D converter, the maximum value is 16383, so 1/5 of that is 3277.
These modules have a 16-bit A/D converter, so I think the default max/min engineering units are meant for compatibility with legacy modules that run in 14-bit mode. 3277/16383 were the min/max for the popular 1746-NI4 analog module in 4-20 mA mode, for example.
I prefer to give up some precision in favor of readability, so 4000 to 20000 is how I set mine up so I can compare the raw values with a multi-meter. I explicitly scale to real "engineering units" like feet/minute or RPM in my controller code.