I didn't make up the figures and they are averages, some cats kill more than 7 and some don't kill any. On the other hand there are some funny people out there, you may be right.
There are upwards of 50 million cats in GB, it is estimated that each cat kills an average of 7 garden birds a year, thats 350 million birds. Besides which they kill all the frogs from my pond (in Belgium).