Given that different conditions produce different crystals (see for instance the description for the "capped columns"), my best guess is simply that each of the 6 sides experienced all-but-identical growth conditions, and thus the correct question is rather why would they be different?
Someone else mentioned 12- and 3-siders, and trying to Google up those images is how I got to that site in the first place. See: http://www.its.caltech.edu/~atomic/snowcrystals/unusual/unus... (Note you have to "-paper" in the Google search, or you'll just be hammered with instructions for paper snowflakes of varying degrees of scientific inauthenticity.)
"While [a snowflake] grows, the crystal is blown to and fro inside the clouds, so the temperature it sees changes randomly with time. But the crystal growth depends strongly on temperature (as is seen in the morphology diagram). Thus the six arms of the snow crystal each change their growth with time. And because all six arms see the same conditions at the same times, they all grow about the same way. The end result is a complex, branched structure that is also six-fold symmetric. And note also that since snow crystals all follow slightly different paths through the clouds, individual crystals all tend to look different."
And see also the next question, "What synchronizes the growth of the six arms?". I'm hitting the limit of what I feel comfortable just copying and pasting into HN.
Given that different conditions produce different crystals (see for instance the description for the "capped columns"), my best guess is simply that each of the 6 sides experienced all-but-identical growth conditions, and thus the correct question is rather why would they be different?
Someone else mentioned 12- and 3-siders, and trying to Google up those images is how I got to that site in the first place. See: http://www.its.caltech.edu/~atomic/snowcrystals/unusual/unus... (Note you have to "-paper" in the Google search, or you'll just be hammered with instructions for paper snowflakes of varying degrees of scientific inauthenticity.)
Edit: Ah, the site addresses that: http://www.its.caltech.edu/~atomic/snowcrystals/faqs/faqs.ht...
"While [a snowflake] grows, the crystal is blown to and fro inside the clouds, so the temperature it sees changes randomly with time. But the crystal growth depends strongly on temperature (as is seen in the morphology diagram). Thus the six arms of the snow crystal each change their growth with time. And because all six arms see the same conditions at the same times, they all grow about the same way. The end result is a complex, branched structure that is also six-fold symmetric. And note also that since snow crystals all follow slightly different paths through the clouds, individual crystals all tend to look different."
And see also the next question, "What synchronizes the growth of the six arms?". I'm hitting the limit of what I feel comfortable just copying and pasting into HN.