This is a double data deal that gives you 4GB instead of their usual 2GB.
A bit similar to what iD Mobile has but for those who prefer to be on EE network*. :)
*iD Mobile = Three
*Plusnet Mobile = EE
The fine read:
> DOUBLE DATA ON THE £10 PLAN
Double data on the £10 Plan. £10 a month for 4GB (instead of 2GB) of data, 1500 minutes & unlimited texts. SIM only plan on a rolling monthly contract
Available to new and existing customers signing up between 29th November 2016 and 4th January 2017.