Think of 'dissimilarity' as pointing out differences; it emphasizes what is not the same. The dissimilarity between the twins was only noticeable in their habits.
Pro Tip 2/3
Not Opposites
Two things can be dissimilar without being complete opposites; they may share some qualities. There's a clear dissimilarity in opinion, though they both value honesty.
Pro Tip 3/3
Subtle Differences
Use 'dissimilarity' when discussing subtle or minor differences that are not immediately obvious. The dissimilarity in their writing styles became apparent only after a close reading.