BUG #15387: explain analyze timing on bug?

The following bug has been logged on the website:

Bug reference:      15387
Logged by:          Zhou Digoal
Email address:      digoal@126.com
PostgreSQL version: 11beta3
Operating system:   CentOS 7.x x64
Description:        

when turn on timing for explain analyze:

```
postgres=# explain (analyze on,verbose on,timing on,costs on,buffers
on,summary on) select count(*) from item1;
                                                       QUERY PLAN           
                                           
------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=254.75..254.75 rows=1 width=8) (actual time=26.938..26.938
rows=1 loops=1)
   Output: count(*)
   Buffers: shared hit=1334
   ->  Seq Scan on public.item1  (cost=0.00..38.00 rows=100000 width=0)
(actual time=0.011..15.674 rows=100000 loops=1)
         Output: i_id, i_im_id, i_name, i_price, i_data
         Buffers: shared hit=1334
 Planning Time: 0.065 ms
 Execution Time: 26.967 ms
(8 rows)

Time: 27.291 ms
```

when turn off timing:

```
postgres=# explain (analyze on,verbose on,timing off,costs on,buffers
on,summary on) select count(*) from item1;
                                             QUERY PLAN                     
                        
-----------------------------------------------------------------------------------------------------
 Aggregate  (cost=254.75..254.75 rows=1 width=8) (actual rows=1 loops=1)
   Output: count(*)
   Buffers: shared hit=1334
   ->  Seq Scan on public.item1  (cost=0.00..38.00 rows=100000 width=0)
(actual rows=100000 loops=1)
         Output: i_id, i_im_id, i_name, i_price, i_data
         Buffers: shared hit=1334
 Planning Time: 0.062 ms
 Execution Time: 11.050 ms
(8 rows)

Time: 11.394 ms
```

why turn timing on increment about 26ms ?