I don't get this.

In Chapter 15 they have this big todo about whether new Christians need to be circumcised or not.  The Apostles decide that it isn't necessary.  

So in the very next chapter I run across this. 

Acts 16:1-5(NKJV)

Then he came to Derbe and Lystra. And behold, a certain disciple was there, named Timothy, the son of a certain Jewish woman who believed, but his father was Greek. 

He was well spoken of by the brethren who were at Lystra and Iconium. 

Paul wanted to have him go on with him. And he took him and circumcised him because of the Jews who were in that region, for they all knew that his father was Greek. 

And as they went through the cities, they delivered to them the decrees to keep, which were determined by the apostles and elders at Jerusalem. 

So the churches were strengthened in the faith, and increased in number daily.

Why did Paul circumcise Timothy when the Apostles had already decided it wasn't necessary?  So what if his father was Greek.  Not sure what that had to do with it.