The first pint will always have a bit more head because the beer will warm in the line causing the beer to shed gas. Probably it's just that; the carbonation should remain the same due to the pressure and temperature of your set-up.
This is your answer. If your lines were chilled as well as your kegs, all of the beer would pour the same.
I'm not sure this is 100% accurate. I serve from picnic taps that are entirely inside the fridge with my kegs. The first pour is always different due to the beer that is in the lines. I don't know if the beer in the lines loses carbonation or what exactly happens, but the best approach for me is to clear the lines (drink or dump) and then pour. After the lines are clear I will get a consistent pour.
I don't pour a pint every day (even not for several days) so my experience may be different from yours if your taps are in regular use.
The reason that your first pour is different is that the beer in those lines are a higher temperature than the beer in your kegs. This is due to many reasons on different systems: vinyl lines insulating from the fridge/freezer air, the air around the lines actually being warmer (especially common in chest freezers but not uncommon in old fridges - cold air sinks & taps are usually higher than the keg), etc. Carbonation is temperature dependent. Carbonation will come out of solution at higher temperatures. Once you pull fresh beer (at the keg temp) into those lines, it takes time to rise in temperature, which is why the first pour can be foamy on many systems. Even a 5*F rise in temperature in the 'line beer' will cause foaming. Stick a thermometer in the beer that you dump - I'll bet that it isn't the same temp as the beer that you drink from the second pour.
Source: I have a fairly involved draft system at home. Long draw, rise in elevation, air cooled remote box, flow control faucets, barrier lines, etc, etc, etc. If there is a problem with a draft system, I bet that I've seen it in my own home. I've had a draft technician over to diagnose the multitude of issues this thing has - it's safe to say that I've learned a lot in the past year.