Why would fibre be slower than ADSL2+?

[#1]

I am struggling to understand the concepts here.  I was happily operating on ADSL2+ for a couple of years and getting around 17Mbps quite consistently.  After a billing problem where I complained to Plusnet, I was persuaded into agreeing to take their fibre service at only £1 per month more - for a 24 month contract.  I've now been using the fibre service for more than three days, and so far my speed test results using various different sites are showing a SLOWER service than the ADSL2+.  I measure less than 16.5Mbps, with a slower upload speed as well.  They say wait 10 days for the service 'to settle down' before complaining, but that takes me beyond the 14 day 'cooling off period'.
While the service is similar to what I had, I'm very unhappy to be caught in a 24 month contract at more money for no gain whatsoever. 
I live on a road parallel to a main through route, and I know the cabinet on that road has fibre to it (at least they put a poster up saying so!).  The cabinet is only 100 yards away as the crow flies, but my telephone line goes to a pole across the street in the other direction, and I don't know the route from there to the exchange.  The exchange is over a mile away.
I suspect that my connection must be direct to the exchange, not to the cabinet with the fibre in it.  If this is so, should I expect any gain in service with a fibre connection? 

[#2]

There is no '10 day training period', there never has been - not even on ADSL. It's high-time that claptrap was stamped out.
Contact PN immediately and raise a fault.

[#3]

Thanks Andrue,  I suspected that.  My call to them is currently on 'wait' time of 30 minutes.

[#4]

That was interesting.  It seems that I am not yet on fibre, despite the arrangement being made a week last Wednesday.  I had a text saying that my fibre was now on line - they have apologised for that and say it is embarrassing.    They also say that there is a problem with my broadband, and the maximum speed I can expect is 24.9Mbps.  As I was promised a MINIMUM of 24Mbps, I now have no confidence that this can be achieved.  However, they have agreed to amend the contract date to that day which they will try to sort this out, giving me 14 days from 9th March to cancel the contract if they don't achieve the target.
It seems the slower service is due to the Hub One being slightly less adept at ADSL2+ than the Netgear router I had substituted for their lousy Thompson one.  I've now cancelled the sale of the Netgear, I have a feeling that I might still need it after all.

[#5]

You're not the first person where the plusnet system has told somebody that their fibre was active when it wasn't. They must be embarrassed a lot.