Clojurians Log v2

Hello! Could someone help me understand why shifting a row followed by some array math with the new row takes almost 3x longer than the time it takes to do the shift and do math on other columns? Is there something special about the shifted column that would cause, in this case subtraction, to be so much slower? This is Tablecloth 7.029.2

(def rand-ds (tc/dataset {:a (repeatedly 1e6 #(* 1000 (rand)))
                            :b (repeatedly 1e6 #(* 1000 (rand)))}))

  (time
    (-> rand-ds
        (tc/shift :b_s :b 1)
        (tc/- :a-b [:a :b])))
  ;; "Elapsed time: 54.576875 msecs"

  (time
    (-> rand-ds
        (tc/shift :b_s :b 1)
        (tc/- :a-b_s [:a :b_s])))
  ;; "Elapsed time: 142.5745 msecs"