Hi
Welcome to the forum.
First of all your ES are negligible, 100 or 300 not worth worrying about.
xDSL is very complex and we aren't always aware of the whole picture. I suspect nearer the master socket you are getting more frequencies in use higher up the scale because it is a slightly better connection for attenuation and/or slightly lower noise. However these higher frequencies run with more errors, you might get a bit more sync speed and along with that comes more errors. If the errors are bad enough on these higher frequencies then bit swapping will likely render them unusable and they will get turned off by the modem, this might see your ES fall after a day or so being connected, it may not of course because you still have the higher sync rate.
xDSL is designed to have errors, it is the only way you can push so much data over decades old analogue telephone wire by accepting a certain level of errors, typically these errors are corrected and hidden from you further up the network stack. Errors are part and parcel of xDSL, you could run with no errors ever but your sync rate might have to be 100Kb/sec to achieve that!
All sorts of other things can come into play as well relating to RF which is a law unto itself. Shortening or making longer the line, even by just a few metres can make a difference to how the signal reflects back down the wire causing more or less constructive interference. Your telephone line is also one big antenna, and making it a bit shorter might make it a better antenna for picking up RF noise in your locality and give you something worse, the opposite of what you might think by shortening the line length.
So overall nothing to worry about I would say, and what you describe happening is simply part and parcel of the dark art of RF and xDSL.
Regards
Phil