Because the horns are in a terrible location (riding about 6" off the ground at the front of the car); and they were not sealed against the elements, they lived perhaps the roughest life of any part on the W113.
I've seen a lot of horns on a lot of W113's, having judged a couple of concours, etc. Never saw an original installed horn. They were all replacements, and often incorrect.
So there are so many horns on these cars it isn't unusual to have them sound different.