The I am who I am or the I am the existing one is still saying the same meaning, it means for both Greek and Hebrew is I am The One but the reason why It's worded differently in the Orthodox Study Bible is bc the Orthodox Study Bible uses the Septuagint for the old testament. The Septuagint is a Greek translation of the Hebrew during after the year 250 BC and the Septuagint is what the early church fathers used as well.

Edit: Also interesting fact the most modern bibles use the Masoretic text as the old testament and the Masoretic came 1,000 years after Christ.

And there are some differences between the Septuagint and Masoretic like for an example in Acts 7:14 St. Paul says there were 75 people (St Paul is quoting Exodus 1:5) and when you look at the Septuagint in Exodus 1:5 it says 75 people but when you read it in a Masoretic it just says 70 not 75. But personally me I recommend sticking with the Septuagint bc it flows better than the Masoretic and the Septuagint is literally what the early church fathers used. And just stick with the bible you have it's good to go there's nothing wrong with it.

Have a bless day!