Regarding wind shape factor,
1. CAESAR II Technical reference states
" ASCE #7 in figure 6-21, A value of 0.5 to 0.65 is typically used for cylindrical section "

2. One of my project specification states
As per BS 6399 Part 2 table 20,
"Pressure coefficient 1.2 shall be applied for Circular section."

Would someone let me advise reasons of different wind shape factors between codes?

I believe that BS 6399 is withdrawn, replaced by BS-EN-1991-4.

Shape factor for acircular section is a function of Reynolds number and can be as high as 3 or more.

See this ZIP files which contain data from two different references.