Why do motorist speed up when you overtake them only to slow down when you're past them?

An interesting observation about overtaking cars.

I've always felt that some (hmm, maybe most) motorist actually speed up when you overtake them on the highway or road. Usually, I don't care much for this practice but lately, I've given this more thought.

Recently, when I was driving home from work I steadily caught up to a motorist on the highway. I was travelling at the posted sped limit and it was a dry road and sunny day. I easily caught the motorist in front of me and over took him only to have him actually speed up. Thus, instead of me overtaking the other driver at my pace, the other driver now keeping pace with me. We were side by side for  a good few kilometres at the posted speed limit (110kph) and then something strange happened he backed off and started dropping back in speed and started to fall behind me. 

This is when I decided to experiment (safely) and also back off my accelerator (There were no other cars using the highway thus, there was no risk). I dropped back to 100kph before I then resumed my pace at the posted speed limit of 110kph. As I continued down the highway I looked in my rear view mirror to see the other driver now quite a distance behind, indicating they had resumed their previous speed as I was going no faster. 

This proved the point to me that: Motorist speed up when you start to overtake them only to slow down when you're past them?

Why do motorist do this?, is it because:
They have some innate need to not be overtaken but give up just as easily
They feel some sort of achievement by 'making you earn' your overtaking move
They want to be "boss" for the overtaking move and their slowing down is their way of indicating "they won" 
Humans have a natural desire to 'be first' 

A strange but it seems regularly occurring observation indeed.
Pete