I have wondered the same.
I had the following from further information from Uno about the Duo product via email:
Duo are new services recently launched and unlike our other services, we don't use just one specific wholesale network for delivery of the service - we use the best value option in that area or exchange.
Duo services have lower assurance rates compared to our other fibre products which is worked in to the cost. It also requires you to bring the line rental to us and is only offered on an 18 month term.
Our Fibre+ and Fibre+ Pro have higher assurance rates so if you're looking for "guaranteed" performance minimums that are nearer to your sync rate, they would be the product to go for.
I posted the following thoughts on the TBB forum:
I believe the assured rate is the rate that they would take any action with their wholesale provider if the actual throughput is lower (than the assured rate).
I may still get what I get now on Duo Advanced, but Uno are going to be in a significantly worse position in terms of escalating to their wholesale network provider if there are performance issues.
For me, it comes down to a question about whether I potentially want to risk compromising my broadband connection to save money (and the saving is not inconsiderable - £14 per month compared to the monthly equivalent of my annual fee with IDNet).
I get the feeling that this is a product designed to compete with the mass market products and may be found wanting by more discerning users, but that is just conjecture.
Don't know how right or wrong I am, but it was sufficient that it made me bottle out of making the commitment.