The surround speaker channels are set individually to -3dB compared to the master channels because the acoustical sum of both surround channels adds up to 0dB if the content is equal. This has its historical cause because the surround channel was originally a mono surround channel with correlated signal for both surround speaker channels, adding up for 3dB at listening position between the two speaker channels. This is only executed in cinema settings. Home cinema systems follow the recommendation of ITU-R BS.775-2 with all five speakers set to the same level. The IRU approach is fitting to modern formats with two individual and therefor decorrelated surround channels which don't add up to a 3dB level raise. The coverage of the surround speakers should be within +/-2dB uniformly over all seating area.