Explain analyse with track_io_timing

What is the unit of I/O Timings in explain (analyze, buffers) ? There is a plan with quite a few nodes.  In each case, the value of I/O Timings is much more than the time for the outer node. A few lines from the plan - 

 Hash Left Join  (cost=14320945.22..7099974624.27 rows=194335062701 width=5511) (actual time=107913.021..108109.313 rows=759 loops=1)
   Buffers: shared hit=738871 read=1549646, temp read=92710 written=92973
   I/O Timings: read=228324.357
   ->  Hash Left Join  (cost=14049069.69..246411189.41 rows=18342148438 width=5467) (actual time=96579.630..96774.534 rows=759 loops=1)
         Buffers: shared hit=684314 read=1377851, temp read=92710 written=92973
         I/O Timings: read=217899.233

At the end, there is 

Execution Time: 108117.006 ms

So it takes about 108 seconds. But the I/O Timings are higher.

Best Regards,

Jay