Why is Pirelli supplying different tyres for Silverstone F1 double-header?

Pirelli will supply Formula 1 with two different tyre specifications in the Formula 1 Silverstone double-header. The British Grand Prix this weekend, which is the second event of the year to be sponsored by Pirelli, will have the C1 compound as P Zero White hard, C2 as P Zero Yellow medium, and C3 as P Zero Red soft. The 70th Anniversary Grand Prix one week later will be one-step softer: so C2 as the hard, C3 as the medium and C4 as the soft. This creates Formula 1 history: the first time ever that there has been two different tyre nominations for a double-header at the same venue.