This stat is useless and misguiding. On most cases you get a low success rate even when the map is easy since most people retry/wait until fail because they missed and they wanted a full combo. Plus you can get tons of fails/retries because FL players, DT players retrying a lot too, same with every other mod. Meaning you can get a really low success rate even on a very easy map.
Then again you got very hard maps like FREEDOM DiVE where the success rate is actually ridiculously high since truth is that map has been cleared no more than 5 times (I'm pretty sure of this), and most clears are because of no fail/half time/easy, which doesn't really make any sense.
Plus, some maps have a very low HP Leech, so even when they may be completely insane, clearing wouldn't be much of a challenge.
Success rate on Another is almost the same as FOUR DIMENSIONS, one being just a very tiring stream map, while the other is by far the hardest map ever ranked.
http://osu.ppy.sh/s/24313 and http://osu.ppy.sh/s/48979
Airman has a higher success rate, and it's obviously WAY HARDER. But:
On Airman most people don't even try to FC and play it just for fun, HP Leech is low so clearing isn't that hard. Meanwhile, Mythologia's HP Leech is higher, you got lots of streams meaning some epic fail on any of those means failing. Plus it's actually possible to FC/easy to get a high combo on so you may get lots of retries/intentional fails after a miss.
tl;dr: Misguiding data and almost useless since it doesn't really say anything, and you can get a better reference of how hard the map is by looking at the top scores.