I feel like its probably a rounding bug, one that might maybe cost you...1 point of damage. But I'd also like to remind you that that isn't how Ashe's passive works; her crits DO do extra damage, she just doesn't crit naturally. Her crit damage scales with her crit chance.
EDIT Yeah, just looked at the tooltip and its worded really stupidly, so let me break it down. When Ashe basic attacks a thing, she applies Frost, which slows. Additional basic attacks count as "crits", dealing 110+Crit Chance damage and doubling the slow from Frost.