Assuming there's a Year 0, there's 19 years to be split between two decades spanning Year -9 to Year 9 inclusive. One of them - take yer pick as to which - only has 9 years. (either that, or Year 0 doesn't belong to any decade thus leaving a 9-1-9 sequence)
If there's no Year 0 then there's only 18 to split so each of those two decades would be generally considered to only have 9 years, yes.
Why? Because like it or not, common perception of decades doesn't match actual math but instead just looks at the figure in the 10s column, thus 2010 is in the 10s, 2020 is in the 20s, and so forth. Which is fine.
Same thing with centuries etc., for all that: I mean, let's face it, we all saw the turn of the century/millenium as being the 1999-2000 changeover because that's when the 100s and 1000s numbers changed, right?