The 10 Arrows 1798 Silver Dollar is an interesting variety in that three of the arrows are missing from the bundle on the reverse. This was not a case of grinding the arrows from the die. Rather, the missing arrows never existed in the first place. Take a look at the comparison image at the top of this page and compare the two bundles of arrows. The placement of the arrows, apart from the missing ones, is identical on both coins. The engraver was given the option of adding the additional arrows, and where to place them. In this case, he simply forgot (or neglected) to add them.
Though not particularly rare, this variety is difficult to find in high grade. The PCGS CoinFacts Condition Census includes only two Mint State examples, the finest of which is a PCGS MS-63.