Assuming this is a bug and not a feature: Ashe's Frost Shot passive does not apply Infinity Edge's True damage. Instead, she gets True damage only when she randomly crits (aka the shots with the x3 slow). Due to the penalty of losing her Infinity Edge multiplicative passive scaling, this significantly reduces her DPS pre-3 items relative to other ADCs.
1. Choose Ashe
2. Buy Infinity Edge and _one_ crit item
3. Repeatedly attack a target dummy
4. Notice that the True damage procs only 60% of the time.