ENGLISH is a very interesting (Read: weird) language. trust me, it really is.
common: something that occurs oftenly.
uncommon: something that occurs seldomly.
this is a double negative. according to the english language rule, a double negative makes a positive. therefore, "not uncommon" should equate to "common". but no.
not uncommon: it is an intermediate situation between common and uncommon. something that occurs seldomly, but not too seldom, and may also be seen oftenly, but not too frequent.
crazy ain't it?
told you it is WEIRD.