In[7]:=
Trace[Table[i*j,{i,1,3},{j,1,3}]]
Out[7]=
{Table[i j,{i,1,3},{j,1,3}],{{i,1},{j,1},1 1,1},{{i,1},{
j,2},1 2,2},{{i,1},{j,3},1 3,3},{{i,2},{
j,1},2 1,2},{{i,2},{j,2},2 2,4},{{i,2},{
j,3},2 3,6},{{i,3},{j,1},3 1,3},{{i,3},{
j,2},3 2,6},{{i,3},{j,3},3 3,9},{{1,2,3},{2,4,6},{3,6,9}}}
In[9]:=
Trace[Table[{i+j},{i,1,3},{j,1,3}]]
Out[9]=
{Table[{i+j},{i,1,3},{j,1,3}],{{{i,1},{j,
1},1+1,2},{2}},{{{i,1},{j,2},1+2,3},{3}},{{{i,1},{j,3},
1
+3
,4},{4}},{{{i,2},{j,1},2+1,3},{3}},{{{i,2},{j,2},
2+2,4},{4}},{{{i,2},{j,3},2+3,5},{5}},{{{i,3},{j,1},
3+1,4},{4}},{{{i,3},{j,2},3+2,5},{5}},{{{i,3},{j,3},
3+3,6},{6}},{{{2},{3},{4}},{{3},{4},{5}},{{4},{5},{6}}}}
同时用乘法和加法做了个3*3表格，为什么乘法的输出结果是单项3*3列表，而加法不是

因为你多加了个大括号