Car: 2001 E430 4-Matic

I installed new Avid ENVigors (235/45/17) on my car on May 30, 2013. Today I replaced the two rear tires, which were worn down to the wear indicators; and in about 2 months will go ahead and replace the fronts, as they still have some life left.

Sixteen months, and approx 25K miles, does not seem like a long lifespan for tires, and I have never had rear tires wear out before the fronts. Is this a fluke or are these tires simply too soft?

(I did nonetheless replace them with new Envigors since they handle well.)

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black