Don't read too much into it - it varies in every pregnancy due to so many different variables: placenta placement, individual temperament, diet, etc. It could be as simple as DC2 is more facing your front so you can feel it more if your DC1 was facing your spine more.
My DC4 (when I was at my most stretched with major diastatis recti) was my least active child, I think DC2 was my most active, but she was facing forward the whole time (was born that way) though DC1 was not far behind her.