The spl measurements are in terms of average and peak levels (85 dB average with allowance for 20dB peaks for speakers, add 10dB to both for LFE) generated by each speaker individually measured at your seat. So each speaker's demands will vary with distance from your seat. If you take all speakers together the average spl at your seat will be greater due to all the speakers' combined output. Generally the surrounds and rear surrounds receive a lesser signal than mains to create the ambient effects of surround, but will also depend on mix (and type of movie to an extent).
The mixes still refer to spl at the seat.
Mixes vary as to how they adhere to such standards, tho. Some will be less, some will be more. Some will concentrate on fronts some will spread it around more...