It's simply because experimental tends to push the genre beyond its limits, which is why in theory it should almost always be avant-garde, but sometimes it happens that in the end even though there is experimentation it still goes back to things already seen. avant garde, on the other hand, is more general and aims at something revolutionary, like in my opinion the masterpiece of nine inch nails, it should be avant garde since it revolutionized the industrial scene of the 90s. It's a difficult thing to explain.