cat A.tsv
1,a,d
2,b,e
3,c,f

$ awk -F ',' -v OFS="," '{print $2, $3}' A.tsv 
a,d
b,e
c,f

• I expected the following 4 commands to give the same results as above:

$ awk 'FS=","; OFS=","; {print $2, $3}' A.tsv 
1,a,d
1,a,d
,
2,b,e
2,b,e
b,e
3,c,f
3,c,f
c,f

$ awk -F ',' 'OFS=","; {print $2, $3}' A.tsv 
1,a,d
a,d
2,b,e
b,e
3,c,f
c,f

$ awk -v OFS="," 'FS=","; {print $2, $3}' A.tsv 
1,a,d
,
2,b,e
b,e
3,c,f
c,f

$ awk -F ',' 'FS=OFS; {print $2, $3}' A.tsv 
1,a,d
a d
2,b,e
 
3,c,f

Can someone explain why the last 4 commands have different results than the first?

Refer: Difference between awk -FS and awk -f in shell scripting - Stack Overflow