Bluegrass?.. lol.. No, not even close.. I think..
Here is the real reason. Many players have Low C-extensions. The E/C string is up on the upper peg and the A on the lower, in reverse of the normal order. These same players also own Basses without C-extensions so to keep things uniform, they string it the same way. Also, some believe the 'E' tension and sound is better with the longer after length to the post. The same is believed by some with 5-string BGs moving the Post itself higher for the Low B.
Here's some pics to fit the text I just wrote;
Normal C-Ext;
C-Ext on a shorter 3-string Scroll;
C-Ext with reversed tuners, G below the E;
3-String Scroll with C-Ext removed with Strings reversed, the E on the A peg;
3-String Scroll modified to 4-string with 4-gears crammed close;
A BG (Smith 25th) with Reversed Tuners for Low B and E above the G and D;
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