Why does a search for a misspelling ("optius" instead of "optimus") find no documents?

gistfile1.sh

echo 'Delete the index.'
curl -X DELETE 'http://localhost:9200/test_products/?pretty=true'

echo; echo
echo 'Create the index. Copied directly from https://gist.github.com/961303 .'
curl -X PUT 'http://localhost:9200/test_products/?pretty=true' -d '
{
   "settings" : {
      "analysis" : {

         "filter" : {

            "3_8_ngram" : {
               "type":      "nGram",
               "min_gram":  3,
               "max_gram":  8
            }

         },

         "analyzer" : {

            "ascii_3_8_ngram" : {
               "type":      "custom",
               "tokenizer": "standard",
               "filter":    [ "standard", "lowercase", "asciifolding", "3_8_ngram" ]
            }

         }

      }
   }
}
'

echo; echo
echo 'Create the mapping.'
curl -X PUT 'http://localhost:9200/test_products/product/_mapping?pretty=true' -d '
{
   "product" : {
      "properties" : {

         "name" : {
            "type":             "string",
            "index_analyzer":   "ascii_3_8_ngram",
            "search_analyzer":  "ascii_3_8_ngram"
         }

      }
   }
}
'

echo; echo
echo 'Index some documents.'
curl -X POST 'http://localhost:9200/test_products/product?pretty=true' -d '{ "name" : "Optimus Prime" }'
curl -X POST 'http://localhost:9200/test_products/product?pretty=true' -d '{ "name" : "Optimus Primal" }'
curl -X POST 'http://localhost:9200/test_products/product?pretty=true' -d '{ "name" : "Fortress Maximus" }'
curl -X POST 'http://localhost:9200/test_products/product?pretty=true' -d '{ "name" : "Megatron" }'


echo; echo
echo 'Refresh the index.'
curl -X POST 'http://localhost:9200/test_products/_refresh?pretty=true'


echo; echo
echo 'Search for "prim". Expect "Optimus Prime" and "Optimus Primal".'
curl -X GET 'http://localhost:9200/test_products/_search?pretty=true' -d '
{
   "query" : {
      "dis_max" : {
         "queries" : [
            { "field" : { "name" : "prim" } }
         ]
      }
   }
}
'


echo; echo
echo 'Search for "optius" (misspelling of "optimus"). Expect "Optimus Prime" and "Optimus Primal".'
curl -X GET 'http://localhost:9200/test_products/_search?pretty=true' -d '
{
   "query" : {
      "dis_max" : {
         "queries" : [
            { "field" : { "name" : "optius" } }
         ]
      }
   }
}
'


echo; echo
echo 'Search for "imus". Expect "Optimus Prime", "Optimus Primal", "Fortress Maximus".'
curl -X GET 'http://localhost:9200/test_products/_search?pretty=true' -d '
{
   "query" : {
      "dis_max" : {
         "queries" : [
            { "field" : { "name" : "imus" } }
         ]
      }
   }
}
'