That’s because I’d learned that decimated means to kill a tenth. The clue’s in the first part of the word – from the Latin decimus or tenth (it came into English via Middle English). There is a specific use of the word that does mean that – in ancient Rome, one in ten of a group of soldiers could be killed to punish the mutiny of the whole group.
But look in your dictionary nowadays and you will see something along the lines of “To destroy a large proportion of something” as the first and major meaning. There may be a little explanation relating to those pedants among us who still insist on the idea of killing only a tenth of the population of whatevers. But this is one that has passed into common usage, and having found this out, I am no longer permitted to froth at the gills when I hear the “other” usage.
I was going to say that I’ll still never use it myself in the less precise way … but I’m not sure that I have ever, actually, used the word …
Be careful! is a series of posts about words that are misused commonly – but really shouldn’t be. It’s not a new variant of meaning, it’s an error that gets duplicated as people see the word misused and copy it.