Minimum Members vs Required Skills

Why are the required skills for some band types more than the minimum members? As in, with certain bands they may have a minimum of 4 members, but need 5 instruments to be fully formed. I'm a bit confused about this, is there an option for 1 person to fulfil 2 skills? Like the drummer can also be the vocalist, for example?
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Initial idea was that there will be musicians with more than one skill, but there are not. At least not for now.
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For rock/metal I needed five members, but I discluded the rythem guitar and it still said fully formed?! I got a rythm guitar aanyway
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