In the U.S., yeast producers sell ADY and IDY at the retail consumer level in packets of 0.25 ounces. Today, I looked at a packet of each, and there is a statement on the ADY packet that says that it contains 2 1/4 teaspoons of ADY. The packet of IDY says that it contains approximately 2 1/4 teaspoons of IDY. Because they are fairly close, I have seen recipes that do not differentiate between the two types of yeast from a volume standpoint. But, you are correct, the two types of yeast are fairly close from a volume standpoint. As an example, if you look at the yeast conversion chart at http://www.theartisan.net/convert_yeast_two.htm
and use 0.58 ounces as an example, 0.58 ounces of ADY is 5.75 teaspoons and for IDY it is 6 teaspoons, or a 1/4-teaspoon difference. However, since people aren't always meticulous in the way they measure out things like yeast (e.g., some use level measurements, some use scant, some use heaping, etc.), and since measuring spoons can have different possible shapes and can be constructed of different types of materials with different manufacturing tolerances, you can see how the difference of 1/4 teaspoon in the above example can get lost fairly easily.