difference between uniformly random subset distribution and $p$-biased distribution

Question

Given a finite set $X$, we denote by $\mathcal U(X,p)$ the distribution over subsets $R\subset X$, where each element. $x\in X$ included in $R$ independently with probability $p$. And $W\subset X$ is a uniformly random subset of size $|W|=p|X|$. Why is the distribution of $W$ and $\mathcal U(X,p)$ different? And what is the difference between them? Thank you very much!