A-10 cannot penetrate modern armor with it's gun. It had difficulty with T-72 when it was designed and could only kill them from the rear.
A-10: 30mm Gatling
M1 Abrams (and other comparable tanks): 120mm (Russland's newest tank has a 140mm gun as much I remember)
That is a huge difference! The only advantage of the Gatling is the high firing rate -- but it is definitively no match for the armor of a modern main battle tank. I also never implied that. When the A-10 was designed, tank armor was less advanced and today the A-10 is mostly fighting against light to medium armored targets. Many countries even in their regular army have still very old tanks in service.
Today's tanks have to take air attacks into account -- with much better top armor but also better armor at the other sides too.
Your calculation is right, I guess, but still 120mm has just overwhelmingly more hitting power than 30mm rounds.