Avos perek 3 - seyag lachachma shtika

Why does the mishna list "x is a seyag to y" - like "giving maaser is a seyag to wealth" but for being quiet to acquire wisdom it says "seyag lachochma shtika, the seyag for acquiring wisdom is being quiet?  It seems the simple answer is that even if you give maaser without any intent to become wealthy, the care you take in giving maaser will eventually lead you to wealth.  But being quiet, which is something passive, won't lead to anything by itself.  This shtika our mishna says refers to your active decision to be quiet because by listening you will hear many sides and let things develop - that is an active shtika, so the order had to be changed.