Can somebody explain why there is a large uncertainty in the mass estimation of neutron stars?
Neutron stars are so small that they are typically too faint to be seen even with the Hubble telescope. Some can be seen as pulsars, but this gives no information about mass. We depend on observing binary systems in which the gravity of the neutron star strip gas off its companion. This spirals round the neutron star in and accretion disc and emits high powered X-rays when it hits the neutron star.
To calculate the mass of the neutron star, we can use Kepler's laws. This means we need to know the diameter of the orbit of the companion. But unless the binary system is fairly close, we won't be able to measure its distance accurately, and without that we can't accurately calculate mass.