Understanding the variation of Arduino's current consumption with respect to the voltage supply
There is a basic error in your methodology which leads to incorrect conclusions.

You cannot determine what is going on from just two data points. Instead you need far more.

Here is a voltage sweep from 5V to 12V with a 0.05V resolution (overkill, but the default setting for the script I use) connected to a Mega:



As you can see the current steadily climes in a pretty linear fashion up to a certain point, then it flattens out.

Everything up to the point where it flattens is below the minimum input voltage for the board. During this time the voltage regulator isn't getting enough voltage, so can't regulate its output voltage to 5V. So the board is running under voltage at this point.  Once it reaches that "knee" point the voltage regulator can regulate the voltage properly - so most of the components on the board start seeing a steady 5V and their current consumption flattens out.

There are a handful of components on the high-voltage side of the regulator - most notably the voltage divider for detecting the input voltage and feeding it to the comparator that drives the P-channel FET to turn the USB power on and off - which will consume slightly more current when the voltage increases, but that is only very small amounts.

So, in short, by comparing a voltage that is below the minimum input voltage for the board with one that is above the minimum input voltage, you are comparing an invalid result with a valid result and inferring that everything in between is valid.