Yes - if you gained pure lean mass, and the amount of BF you had remained constant then the overall % that this body fat would make would go down.
ie:
Say you were 180 pounds
You had 160 pounds lean tissue and 20 pounds fat
At that point your BF% = (20/180) x 100 = 11.1%
Now you are 182 pounds
You now have 162 pounds lean tissue and 20 pounds fat
So you now have a BF% = (20/182) x 100 = 10.98%
As you can see - it is not a huge difference, but it all counts eventually.