Style and sometimes materials I think. Some might have different hats or expressions? I haven't really looked at Fumos in like 5 years.
But you can easily see some differences between Cirno in these two: >>13308>>13310
-The central bangs are shaped differently
-The red ribbon is different
-The eye shape seems different, but it might be perspective
-Hair ribbon might be different, it's hard to tell from the front, but they both look completely different from the one in OP
Cirno has a very simple design and that's a fair amount of changes just between two versions