Cloning disk with dd writes twice what it reads?

[Basic75]

While cloning a disk with dd(1) I checked on the speed with iostat(8) and got a weird result.
The MB/s written to the target, disk4, is consistently twice the MB/s read from the source, disk5.

> iostat 1
              disk0               disk4               disk5       cpu    load average
    KB/t  tps  MB/s     KB/t  tps  MB/s     KB/t  tps  MB/s  us sy id   1m   5m   15m
   17.14   28  0.46    16.00  141  2.20    16.00   71  1.10   5  3 92  1.80 2.09 1.98
  352.00    1  0.34    16.00 8044 125.68    16.00 4022 62.84   2 10 87  1.80 2.09 1.98
    4.00    1  0.00    16.00 7970 124.53    16.00 3985 62.26   2 11 87  1.89 2.10 1.98
    0.00    0  0.00    16.00 7950 124.21    16.00 3975 62.11   2 10 87  1.89 2.10 1.98
   32.50   16  0.51    16.00 7961 124.40    16.00 3981 62.20   2 11 86  1.89 2.10 1.98
   13.07  146  1.86    16.00 8009 125.15    16.00 4005 62.57   3 13 84  1.89 2.10 1.98
    0.00    0  0.00    16.00 7947 124.18    16.00 3974 62.09   2 10 88  1.89 2.10 1.98
    0.00    0  0.00    16.00 7885 123.21    16.00 3943 61.61   2 10 88  1.82 2.09 1.98
    0.00    0  0.00    16.00 7960 124.37    16.00 3980 62.18   2 11 87  1.82 2.09 1.98
    0.00    0  0.00    16.00 7939 124.04    16.00 3970 62.04   2 11 87  1.82 2.09 1.98
    0.00    0  0.00    16.00 7969 124.52    16.00 3984 62.25   2 11 87  1.82 2.09 1.98

How is this possible?