Allowable load tables based on usage
Loads are based on use...not all loads are the same. 

Floor loads are considered permanent loads (100%), while snow loads are considered short term loads (periodically loaded).  Snow in some areas are rated very short term, less than 7 days (125% stress). Some snow loads are rated short term, less than 1 month (115% stress).

What this means is that you’re allowed to increase the allowable stress by a factor of 15% or 25%, respectively. 

Little known fact: when a single joist/beam is calculated, allowable stress values are less than if they are repetitive. That is to say, joists or beams installed repetitive (say 16” o.c.) can use a higher allowable stress value than a single joist/beam.