Ive really tried to do some digging on this but I cant find anything concrete.
The only obvious is that when migrating from FTTC -> FTTC then there is a transition period of downgrade to adsl, due to there not being any migration path yet for vdsl -> vdsl.
Despite all the digging, I cant find anyone who mentions going on BT adsl -> BT infinity.
Theres a vague reference
here where it was offered as a possible explanation for the OP loosing his Be connection early... but seemingly confirming that the normal process is to go from Be straight to Infinity.
One thing does strike as a possible reason though is that its not unknown for BT to accept the order and the cab not quite be ready.
OR something that can seemingly be common in newly enabled areas.. the FTTC cab is fully subscribed and waiting an upgrade, which normally doesnt take too long before they add more capacity - usually a couple of weeks, in which instance the EU would stay on adsl.
It would be interesting to know for sure.