Lulu Silence Doesn't work?

I have noticed that Lulu's silence doesn't really work... I silenced a Garen while he was doing his spin and he continued to spin and do a ton of damage, even though he was in squirrel form. Another time my silence didn't work, was when Alistar looked as if he was going to head-butt, but I silenced him and then he continued to dash forward and knock my ADC back into a wall. I feel like the silence should cancel these abilities, but it doesn't. What is the point in a silence, if people are still going to be able to use their abilities? If anyone could explain, if the silence was made this way on purpose or not, I would appreciate it.
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