I had experienced increases in speed of the same magnitude on my BT pre-Infinity service, right up until I switched over.

BT had no explanation for this either. I also use speedtest.net, amongst others such as speedtester.bt.com and thinkbroadband.com

I speculated that perhaps as the number of BT users moved away from the copper based service to the fibre optic based Infinity, there was less contention on the copper based service and thereby the non-Infinity users were being benfitted as a by-product of the migration.

But you know what speculation gets you.