Think of it as a continuum:
- Very uncommon
- Not Uncommon
- Very Common
Then the interpretation of 'not uncommon' means more often than 'not uncommon' but perhaps not 'common'.
But even that sort of view requires reader and writer having an agreed upon scale and what the gradations mean. Without that, the writer and the reader may quantify the territory covered by 'not uncommon' differently.
Another way to view it is that uncommon by itself means 'not common' so 'not uncommon' could be written by extension as 'not not common'. With the double negatives cancel leaving you with 'common'.
It's a poor choice of words because 'not uncommon' doesn't tell you what the prevalence of the condition described is, it merely tells you what it is not. At a stretch, it could mean 'very uncommon' (which also isn't precluded by 'not uncommon' literally. It could also mean 'in all cases' (because that also isn't precluded by 'not uncommon'). Its interpretation is very much dependent on the reader and that isn't desirable in clear communication.
It would have been better to say 'common' or even better to have cited some sources that could put some actual quantization of the frequency of the state discussed.