I actually believe that there is more to this. I just started tracking these differences so I could be wrong but two games I checked out had bigger differences in odds as the kick off was coming nearer.
My working theory is that odds start with small difference due to different margins. Than users start betting on more popular markets causing them to drop more on favourite side. It seems that odds adjusting algorithm doesen't track wages across all same markets. Instead it works only on individual odds.
Now as I write this I realised that if my theory is correct the odds for favourite and underdog should be moving in different directions.
*In my post when you see word favourite it actually means market outcome that is getting more money, not the team which is more probable to win. Vice versa for underdog. Maybe there are better terms to use but it is still very early in my part of the world and my brain still needs some time and coffeine to hit working temperature.