An attacker from below is harder to spot and also to elude. Think about it - for many reasons, a shark deeper than its potential meal is less visible than a shark swimming on the same plane. The greater the angle from and the further below the shark is from a seal means the seal is going to have a tough time seeing it before it becomes a statistic in a South Farallon Island study.
At the same time, a seal's best approach to evade the GW is to bust a move quickly in the opposite direction of the shark's attack. However, a seal attacked from below can realistically only travel up . . . and there's only so far it can go in that direction before the shark sinks it's teeth in.