Not at all. The use of negating of a negative term is called a litotes. It is generally used to understate the affirmative position.
To say they are 'not dissimilar' means to me that they have a lot in common, but aren't quite close enough to considered similar. Close, but not quite there. Another commonly used litotes is 'not uncommon'. It's not uncommon for people to use the phrase 'not uncommon'.
Good Luck
--------------
To get the most from your Tek-Tips experience, please read FAQ181-2886
As a circle of light increases so does the circumference of darkness around it. - Albert Einstein