You know - I think I'd keep a record of that for a few weeks. Just once a day, measure both arms. Then take it to my doctor and ask "why is it different in each arm"? However, I would try not to worry about it so much. (I'm terrible at taking my own advice, but it all seems so clear when talking to other people!)
The numbers are pretty though - even the 90 is "doable" considering it's often lower
Oh - another interesting experiement might bo to take it in the right arm FIRST then the left arm. That way, if you notice a trend that it's just the first reading that's higher, it may just be an anxiety thing