This is a pretty explanation of how Maple operates.
But the CDF of a random variable X is, by definition, a non decreasing function of x in the range x = ]-infinity, +infinity[.
For a continuous random variable it is even a strictly increasing function from the support of X to [0, 1]. So each p in [0, 1] has a unique inverse in the in the support of X ("support" is important xhen the CDF exhibits a plateau : in such cas the inverse image of p can be a an interval).
The CDF here is a strictly increasing function from -5 to 5, as CDF(X, x) shows.
Rigorously Quantile(X, x) should return a single value, not 5 values !
If it does so, this is because Quantile(...) does not work on the true CDF, but on the expression it has over the support of X.
Doing this, Quantile ignores what the CDF is outside this support, and naively extends the CDF outside of [-5, 5]
I think this is a rough error in the implementation of the Quantile function.
The piece of code below avoids the problem in the present case.
evalf( eval(%,p=0.2) );