Naftiridine hydrogenation

I was reading an article about the synthesis of 1,2,3,4 tetrahydronaphtyridine and I found this image. [![enter image description here](https://i.stack.imgur.com/NyDJA.png)](https://i.stack.imgur.com/NyDJA.png)
I don't understand why the major product of hydrogenation is the less substituted and not the most. I thought that with an alkyl chains there was more negative density on the ring and so hydrogenations was most likely to happen in that ring.


