I hate to use NTRP rankings as an example, but here it goes anyways:
I think some people are under the impression that as you move to a higher NTRP ranking, that the gap between you and someone else shrinks, not true. The difference between a 5.0 and a 7.0 is the same as that between a 3.0 and a 5.0. That's a 2.0 difference either way.
I'd say I'm a 5.0 player, I've beaten quite a few low to mid-range level d1 players, and I've hit with players who are top 1000: jarmaine jenkins and oren mativassal, luke jenson (2 years ago, he's still very good), tim neily too, (not sure how high he got ranked though). If you think a top 1000 player in the world is going to hit winners off any shot that I hit, your wrong. If you think I'm not going to win one point, your wrong. What will happen is I'll loose 0 and 0 and I'll get a couple duece games. It will be a blow out and it won't be close at all, but they won't hit winners off every ball I hit.
Another thing that needs to be stated is that ITF rankings (especially junior ITF rankings) are very biased. The point system puts a lot of value on how many tournaments are played and not enough emphasis on results. For example, a local country club that I used to work at hosts a men's open tourney, the winner gets about 5k, so obviously it attracts some big names: the person who won it peaked at top 200, was currently a top 500 player, and actually had a win over fed in '01 and a win over safin in '02. The second place player was jarmaine jenkins, and I believe his atp ranking was around 850 at the time we made the draw.
Anyways, in the early rounds we saw a top 25 nationally ranked junior (18 year old) beat another 18 year old who was ranked top 50 in ITF, relatively easily (3 and 3). We saw the top 50 ITF ranking, gave the a seed, but didn't look at his match results. After the match, we went back and looked at the kids record. He had so many early round losses, but played so many tournaments, his ranking sky rocketed. Since then, we started putting much more effort into seedings, looking at head-to-head match ups, rankings, career high and recent results.
Moral of the story is that rankings are decieving and in order to truly predict who the better player is one must find as much information as possible.