What does it mean if there are 2 different figures for a dimension on a blueprint?
The two numbers represent the minimum and maximum range for the dimensions shown.

The dimension units would normally be specified on a drawing like this. In this case you can make two inferences that the dimensions are in inches as follows:


The stud/thread size is shown as 1/4-20 which is a standard inch size fastener.
The dimensions of the head size for a 1/4-20 fastener fall in line with what you would expect for such a fastener. For example a head diameter in the range of 0.443 to 0.472 inches is logical for a 1/4 inch diameter fastener.


As the OP has noted the reference to the ASME B18.6.3 document is one that specifies dimensions for standardized fasteners. Taking a quick look at this document is problematic because it is a controlled distribution document that requires a sizeable fee just to acquire a license to see it for just personal use. After quite a search I located a web page that showed a snapshot of one page of this document as shown below:


(image from: https://www.coursehero.com/file/24235851/ANSI-B1863-Machinery-Handbookpdf/)

Dimensions are in inches as one would expect for an ASME standard.

A screw head size chart from another vendor is shown below which some of the dimensions on the OP's PDF picture from Fastenal as min and max values. 



(image from: http://homepages.cae.wisc.edu/~me231/online_notes/dimensoning/fastener_handout.pdf)