In addition to the points /u/EsterWithPants made, some champions just have higher winrates at lower elo than higher elos. For example, Aphelios is 50% winrate at masters and only 46% winrate at silver. This is due not only to the difficulty of the champion, but also the coordination burden of their playstyle. Taliyah is another example, her winrate at silver in mid-lane is 49% but goes up to 52% as you get to higher elos.
Inversely, Annie, a simpler champion, is higher winrate at lower elos than higher ones. That tends to make a compelling argument for why simpler champions should be played if you're intending to climb from bronze, silver, etc.
If you want to check these stats for yourself, check sites like ...
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