Because the metal detector is not doing a metallic analysis. It is inferring what the target is based on the phase shift, strength, size, and shape of the magnetic field that is induced into the target by the transmit coil (and subsequently detected by the receive coil). Electromagnetic properties of metals like conductivity and magnetic permeability (which determines magnetic field strength and phase shift of the field induced in the target and "coupling" with the receive coil) and target shape affect the sensed magnetic field of the target which the detector uses to "ID" the target, but it is only a guess as many external factors such as soil type, nearby targets, and target corrosion and damage can distort the field from the "ideal". Also, it just so happens that aluminum pull tabs, nickels, and gold rings of similar size are a very similar in these electromagnetic properties as far as the detector is concerned and the phase shifts are similar, so most metal detectors have trouble differentiating between these electromagnetically similar targets. Higher conductors like copper and silver are much easier to differentiate because they result in large phase shifts, iron is also easier to detect and discriminate because it is ferro-magnetic and this affects the magnetic coupling to the receive coil which can be easily determined by the detector (though field distortion can cause falsing). Some multi-frequency machines like the Equinox can eek out some additional information as the target is hit with different frequencies and behaves slightly differently to each of those frequencies such that there is some additional capability to differentiate between a nickel and a pull tab but corrosion and soil conditions can still fool the machine.
Bottom line: it's complicated, but these mid-range conductive metals like brass, lead, aluminum, gold, and nickel of similar size are hard to differentiate from each other.
