why is the reader called twice, after the query params get modified?

core.cljs

(defmulti read om/dispatch)

(defmethod read :input-handler
  [{:keys [query state]} k {:keys [query]}]
  (println "read: input-handler")
  (let [st @state]
    {:value {:input-handler (clojure.string/join [query "+X"]) }}))


(defui SomeInput
       static om/IQueryParams
       (params [_]
               {:str ""})
       static om/IQuery
       (query [_]
              '[(:input-handler {:str ?str})])
       Object
       (render [this]
               (dom/input
                 #js {:key "search-field"
                      :value (:str (om/get-params this))
                      :onKeyUp
                      (fn [e]
                        (om/set-query! this
                                       {:params {:str (.. e -target -value)}}))})))

(def reconciler
  (om/reconciler
    {:state init-data
     :parser (om/parser {:read read})}))

(om/add-root! reconciler SomeInput (gdom/getElement "app"))