Why was I charged with 2 different counts in my DUI case?
You were arrested for drunk driving, nothing more. You show up to your first court appearance and learn that the DA's Office filed not one, but TWO, DUI charges against you. First of all, can they do this? Yes. Secondly, why did they do this? Put simply, the reason the DA charges both crimes is to increase the chances of convicting you of at least one offense. Both charges relate to the same act but are alternative theories to convicting you of DUI.
Imagine a 250 pound guy who can drink many alcoholic beverages without it seemingly affecting him. Let's say he got pulled over and his blood alcohol content (BAC) was .09% (over the legal limit of .08%). What if he performs really well on the field sobriety tests and felt perfectly fine to drive? Then the DA may struggle to prove he was, in fact, under the influence of alcohol in violation of Vehicle Code section 23152(a)(Count 1). BUT, the DA can nevertheless prove that he's guilty of having a BAC that's .08% or more, a violation of section 23152(b)(Count 2).
Now imagine a 130 pound guy who's a "lightweight" when it comes to drinking. After just one stiff drink, he feels the effects of alcohol and shouldn't be driving. Say he got pulled over and his BAC was only .05% (well below the legal limit) but he performed terribly on the field sobriety tests and showed clear signs he shouldn't be driving. Here, the DA wouldn't be able to prove that he's guilty of Count 2 (having a BAC of .08% or more), but may still be able to prove he was under the influence of alcohol, a violation of Count 1.
In sum, prosecutors file both charges to cover both scenarios and increase the odds of convicting folks in cases that proceed to trial.
One bit of good news if you find yourself convicted of both charges is that at least you won't face double the punishment. That's because you cannot legally be punished where the two charges stem from the same conduct.