CM is not centimeters! It's "Circular Mills" 1" square is equal to 1,273,237 Circular Mills. The ampacity tables I've been looking at list the cross section area in CM (thousands). Multiply the listed number by 1000 and you get the full number.
A 1/4" x 1" bus bar is 318.3 circular mills (thousands). Actual circular mills is 318,300 (318.3 x 1000). 318300 / 1273237 = .25, or 1/4".
Here's another bus bar ampacity table. The benefit for this table is that it has aluminum and copper side-by-side. Note that the table is for 6101-T6 not 6061-T6.
I found this really helpful but I do have a question: What does it mean by "cross-section area"? It doesn't seem to be a reference to the bar's cross section...or at least a bar that's 1/2" wide and 1/8" thick does not have a cross section of 79.6 cm (squared? it doesn't say). Should I just be ignoring the whole "cross-section area" part of it?