Most of history's great sporting rivalries have been pretty closely contested. Take Muhammad Ali versus Joe Frazier, for instance. That ended 2-1 in favour of the former, with their final encounter going to the 14th round.
In general, pro-wrestling has followed largely the same trend. From a marketing standpoint, it just doesn't make a whole lot of sense to have one competitor completely dominate another, particularly if you're planning to spread the feud out over several pay-per-views. Fans will simply lose interest if it's too obviously one-sided.
That doesn't always happen in WWE, though. Over the years, several of the company's biggest rivalries have ended up being a little asymmetrical, either in terms of the official match score-card or just the general tone (for example, if one of the two men or women involved is humiliated in promos at every available opportunity).
And before you ask: yes - at least 75% of the time, the beneficiaries of these lopsided feuds are exactly the kind of names you expect to see a W next to. We're talking John Cena, Triple H, Brock Lesnar and The Undertaker.