Why does package guarantee's available not match the guarantee I get at checkout?

[levimineur]

How come I am offered to pay for a package of upto 134mbs claiming a guaranteed speed of 100mbs then when I get to the checkout it says I'm estimated to get between 23 - 30mb with a guarantee of 18mbs? 

[pippincp]

Because the upgrade needs an engineer to perform as you'll be changing from a VDSL service to either G.Fast or FTTP.

Give the Ultrafast team a ring on 0800 587 4787.