I'm not sure I understand. This looks like pretty standard floating point error. All the answers seem reasonable.
What version are you using? I get the same results for Surv2 and Surv3 with 10.4 on my machine.
If you want, you can use N as follows to get the right result:
sigma = {{2, -1/2}, {-1/2, 2}};
Dis = MultinormalDistribution[{0, 0}, sigma];
N[SurvivalFunction[Dis, {20, 20}], 20]
7.0820200214939363143*10^-120
