当我的输入为0.1时，为什么我的答案给出1.49012e

Question

#include <iostream>

using namespace std;

class deci {
    float a;
public:
    deci(float b)
    {
        a = b;
    }

    void operator-()
    {
        a = a - 0.1;
        cout << a;
    }
};

int main()
{
    float a;
    cin >> a;

    deci b(a);
    -b;
    return 0;
}