makePlus1 doesn't return an argument, but returns a function that does. makePlusN takes one argument, and returns a function that itself takes an argument.

Results

function makePlus1() {
    return function(x) { return x + 1; }
}
log(makePlus1()(10));
function makePlusN(n) {
    return function(x) { return x + n; }
}
var plus10 = makePlusN(10);
log(plus10(100));