Stroke order according to Nciku and Visual Mandarin for 妆 and 装, the left radical, 丬, the two ticks go before the bar. But in 乘, the radical that looks like 丬, the bar comes before the two ticks.
Why are the stroke orders different? They're practically look the same.
As @deutschZuid mentioned in his comment, the component in 乘 that you mention is technically not its radical. The "radical" refers specifically to which component the character is classified under in dictionaries, which in this case is 丿.
But anyway, to answer your question, the 丬 component in 妆 and 装 and the component that looks like that in 乘 have different stroke orders because they were originally derived from different components in the first place, even though they may look similar here.
In the case of 妆 and 装, their 丬 radical comes from 爿, in which the vertical bar comes last in the stroke order. In the case of 乘, the component that looks like 丬 is actually from 北, in which the vertical bar comes first.
[As an aside, the derivation of 乘 is actually even more complicated. It actually was originally 椉, which ended up getting simplified into 禾 + 北 = 乘.]
