Tuesday has been cancelled

I got this SMS text yesterday from T-Mobile: "The 10 days free web'n'walk we gave you for topping up 7 days ago has now expired". Now, I admit I'm not a big mobile phone expert, but I thought I understood what English was. How does 10 days of something only last for 7 days? No matter how I figure it, somehow, somewhere, T-Mobile have taken back 3 days free stuff off me. Any ideas? Is there something I'm not getting (apart from those 3 promised days?) Is it simply that T-Mobile, like most big companies, are charlatans?