Rankings isnt the only way to decide who is better, Bovina has had injury problems and illnesses the last 2 years while Dementieva has played full seasons.
There's 2 ways to look at this:
Against the rest of the tour, Dementieva has the advantage. Dementieva is arguably the best rally player in the game. Most players on tour cannot properly attack her serve, and dont have a powerful serve game, and if that's the case, Dementieva will likely beat all those players. Which is why she has success against much of the current Top 10, because they're more defensive-minded, or just can't return her ugly 2nd serve for some reason (Davenport). While Bovina is your run-of-the-mill brainless bash-n'-smash player, but not the best at it, so the top players will usually play the percentages and win 2 out of 3 sets.
Head-to-head, Bovina has the advantage. Bovina dictates every point when they play. The result simply comes down to Good Bovina = Win, Bad Bovina = Loss. Also, take in account if the surface is slower, which would enhance Dementieva's chances.