For a given SPL, it's likely that surrounds produce more distortion than mains (or at least more distortion than the LF and RF speakers). Distortion can sound like "loud," to the brain. Sensitivity ("efficiency") of surrounds is liable to be lower, so they need more power in watts to produce a given SPL, and their power handling is often lower than that of the mains. Some manufacturers succumbed to this cost and size saving temptation in the early days of Home Theater, when it was hoped that the highest SPL would be reserved for the sub and the front channels (and it still generally is). But mixers and filmmakers have their temptations, too, and very high SPL above subwoofer range gets in to surrounds. I'm thinking of The Grey, gun and cannon fire, rockets taking off, etc. in a number of films, and the leading edge -- above sub crossover -- of many special effects. Could there be a worse combination of variables? Yes. Add in user reluctance to use high crossovers, as high as 110 or 120 Hz, even when Audyssey recommends them (often when surrounds are deprived of boundary gain in the room, and had a high F3 even in an anechoic chamber), and when DEQ not only turns up the bass (above crossover to sub) but also turns up the overall SPL of the surrounds, as it does.