Difference Between Single Drive and Dual Drive Modulators

Hello

In OptisSystem there are two kind of modulators that are giving me problems:

Dual Drive MZ Absorption-Phase
Single Drive MZ Absorption-Phase

In theory Dual Drive modulators are better than Single Dive modulators because they can control chirp frequency that Single Drive modulators no. My problem is in simulations, both modulators behave similarly, and there are cases where the single drive modulator performs even better in the same scenarios. Please if someone can explain the reason for this behavior.

If the both modulators behave the same, why bother to tell them apart?

Thanks