there's a couple of threads open here about this. I did get some kind of explanation but can't find it right now - - - like you, applied for the £15.99 jobbie but have to accept the £19.99 one. Still waiting on installation so hope I will be as pleased as you. Again, like yourself, I was a 'doubting Thomas' as to validity of their advert but I do believe there are people out there on the slower speed.
Thank you for contacting Tiscali Customer Support.
Most other ISPs use a third party network to deliver broadband to their customers. Tiscali has set up its own broadband network in areas where we have large numbers of customers. This means that we can offer lower prices to our customers as a result.
The Tiscali Broadband network currently covers around 75% of broadband enabled homes. In areas not covered, we use a third party network to deliver broadband to our customers, in these areas our costs are higher and as a result we cannot offer the £15.99 product. However our 5 X faster product is still up to £10 cheaper than what most other ISPs can offer.