Upload speeds 20Mbit/s limit? why...

[#1]

This is more of a curious than anything. I always thought the 20mbit/s upload limit is because that's what the line could support but according to the Openreach Engineer most fibre lines actually support up to 33mbit/s and the lines are only at limit capacity (he showed me the results on his equipment)
So my question is, how come most ISP's cap the speeds at 20mbit/s when you could give almost another 50% is there a reason for this?

[#2]

It's not us capping the line! We can order three upstream speeds on fibre - 2Mb/s, 10Mb/s or 20Mb/s, those are the options available to us when we're placing orders. I dare say there'll be other speeds available for us to resell when more testing has been done, but as things stand those are the options we have.

[#3]

Sorry, please, I never meant to imply that it was you guys capping it, I was just wondering why the Wholesellers don't sell a high speed.

[#4]

BT OR's products and wholesale prices  >  http://ipsite.org/1rvw

[#5]

Could be some technical reason
Same applies to the download speeds - they are capped at a sync of 80Mbps however those next the the cabinet should be able to get 100Mbps if their line was allowed to run at the full unfettered speed of the 'profile 17'  that BT is running on its Dslams.
I speculate now
Maybe they have found that the higher upload speeds if permitted bleed though on the frequency spectrums into the download frequency allocations and make the download unstable.  As FTTC is still in reality a new product in field trials BT don't want this sort of variable thing to muck up their analysis programs on line settings.  Maybe when they have enough data to know when the line is genuinely unstable and not just unstable from upload instability issues they might release/uncap the down and uploads to the max the profile will allow.
Another idea - maybe they have found the uncapped up/down service adversely affects other nearby lines, so rather than letting the few have an A1+ upload connection at the expense of everyone else on the street BT are giving everyone the best overall service possible.
Note that the above are just my guesses....