I will attempt to address this. As to specific gravity, there are things called inclusions which are differing substances inside crystals. Those inclusions are in space in between crystals? That changes the specific gravity. So would any flaw in the crystal lattice of any crystal.
Also, ruby and sapphire are corundum. They are both corundum but they have differing Mohs and specific gravity measurements. The differing colors are caused by trace elements. Some rubies have more of the trace element, some have less, so there will be a range. They tend to be very similar in specific gravity but inclusions and other elements within a stone will change both measurements. Imagine, then if you will, every diamond is compressed carbon. Some is compressed at higher pressure than others. That diamond formed under different circumstances than another, it only makes sense they would differ. Within a range, diamonds are generally between. 9.1 and 10.
Look at it another way. Copper is copper the world round but no copper is as pure as the copper from the Upper Peninsula in Michigan. When they analyze it they can tell where it comes from simply by its purity. Many of the emeralds they can tell by the color where they come from, even the turquoise in the southwest they can distinguish which mine the turquoise came from. All these substances are similar to others from around the world. The distinctions come from the similar conditions under which those substances were formed such that people can distinguish them just by their singular characteristics. When considering Mohs measurements and specific gravity the same kind of variances are seen.
Ranges are more accurate since every stone is different.