Cointrackers doesn't appear horribly inaccurate, but they're offering the broadest-possible generalizations in an area where the most subtle differences can mean huge changes in value. Coins are graded on a 70-point scale, and there are some issues where a difference of 5 points could mean a thousand-dollar jump in value. With the coin you posted, that could mean just a little bit more hair detail, and a little more on the wings. 1891-O is not at all a rare issue - in lower grade like yours is, it's worth only a little above melt value. However, it's one of those which are quite scarce in higher Uncirculated grades, like so:

That is a $5000 coin. There are issues like 1893-S where a barely-readable example will still set you back no less than $1000 (and one in the condition of yours would be worth over $5000), and other issues where a coin in the condition I posted above won't cost you $100. It's a whole lot more complex than Cointrackers makes it seem.