Re: @JB: It's Worse Than That
I assume (I didn't check) that the DVD was the Steam client, the installer and the game program binaries. The download was probably updates and game data. OR the online version was newer so it just overwrote the DVD version.
I've seen similar behavior with a Mass Effect 1,2 & 3 boxed set (3 DVDs). It ignored the DVDs and just downloaded the games. 16Gbytes total with a later 4.5 Gbyte update for ME3.