Ice ratings can be nebulous, because the medium constantly changes.
When a waterfall first freezes each season, the ice is thin and the climbing desperate. As the ice thickens, it becomes easier to climb and protect. In early morning it's probably cold and brittle, though it might turn to perfect plastic ice by mid-morning, and slush by afternoon. Other factors change a route from year to year, like amount of runoff, prevailing temperatures, and wind. Far more important than a rating in a guidebook is the climb's appearance, the condition of the ice, the temperature, and the climbers.