FTTC faster and more reliable on extension than master?

Hello.
I have BT fibre 1 FTTC in Canterbury area.
I get full speeds and very reliable connection. My modem however always shows connection 'interleaved' in download and fastpath in upload. No problem though, works great.

Yesterday, before I had to move the extension, I moved the modem to the master socket (new BT installed master socket - done in 2017). The moderm shows now fastpath connection also in download but the connection is noticeably slower (speed test shows a consistent difference of -10mbps).

It's not really a problem since I have to plug the modem on the extension anyway - but just curious why this happens? Wasn't fastpath supposed to be better than interleaved? Wasn't a master socket supposed to be better than an extension?
Thanks!