Jury duty...is this normal? or unusual?

I'm 22 & just got a letter in the mail asking me to fill out a form stating if I was available or not for jury duty in July on a Thursday. This is the first time I have ever been called for jury duty so I don't really know how this works. I checked off that yes, I would be available. The asked some other questions (I can't really remember what mostly just occupation? for which I put student because I just graduated college, don't have a job yet & am started law school this September, and something like are you physically and mentally able to be a juror? for which i said Yes, and more of those types of questions). I mailed it back the same day. Now, a couple of days later I get a letter saying I have been excused and do not need to show up. Is this a misunderstanding in that they think I won't be available? I don't want to get in trouble later down the road because they thought I said I wasn't available when I was...should I call them and tell them I'm available or is this something normal? Thanks.