Drums sound really strange without reverb. If you've ever drummed outside, the difference is really stark - I play outside with my samba band a lot, and all the drums sound really cold and quiet, so you end up playing a lot harder automatically just to get the sound of your drum to be as loud as you're used to. I imagine the anechoic chamber might be similar, with the added effect of not hearing any drum sound which would normally bounce off the floor - do, for example, you'd probably hear very little snare wire sound.