AWR , Est Physical Reads in SGA Target Advisory

On my AWR report

in Activity Statistics,  for a Activity Statistics at 1 I have a est Physical Reads a 3,919,446,992.

but in  Instance Activity Statistics the physical reads is on ly at 18,458,840.

why this difference ?

SGA Target Advisory

SGA Target Size (M)	SGA Size Factor	Est DB Time (s)	Est Physical Reads
14,080	0.25	27,619,843	23,941,550,006
21,120	0.38	20,511,057	13,390,398,703
28,160	0.50	18,719,378	10,731,837,809
35,200	0.63	16,345,544	7,207,863,018
42,240	0.75	15,516,115	5,976,764,718
49,280	0.88	14,488,866	4,452,491,783
56,320	1.00	14,129,965	3,919,446,992
63,360	1.13	13,488,465	2,967,805,262

Activity Statistics

 

Statistic	Total	per Second	per Trans
db block changes	14,274,971	3,962.62	39.96
execute count	9,351,289	2,595.85	26.18
logons cumulative	582	0.16	0.00
opened cursors cumulative	2,242,449	622.49	6.28
parse count (total)	2,240,140	621.85	6.27
parse time elapsed	4,957	1.38	0.01
physical reads	18,458,840	5,124.03	51.68