"Base reaction" different from sum of join reactions

[steveingit]

I've performed a response spectrum analysis with Sap2000 v8.3.5
I've modelled only ten concrete shear walls, seven story high (no columns or beams modelled).
At each floor I've defined a master node with lumped mass.
At each floor, each node is constrained with rigid diaphragm (so I've 6 different diaphragms).
Only 7 shear walls are fully restrained at base (u1,u2,u3,r1,r2,r3),3 shear walls are only constrained
for translations (u1,u2,u3).
After analysis performing, I've read "Base reactions" from "Structure output" table: base reaction I read here is 827KN (x direction).
Then I've exported to excel "Reactions" table from "Join output" menu, but the sum of these reactions
in the same direction (X) is about 1030KN, there's a difference of 203KN!!
The same happens if I fully restrain all base nodes.
What happens? Anyone can help me?
Thanks in advance,
Steve.