questions about the output of "gee" and its summary

Dear R-helpers,

I am using the package "gee" to run a marginal model.

Here is the output.
In my simulated data, both x and z are time-varying, so I include their
interaction terms with time indicator (i.e. tind=0, if time 1, and 1 if time
2)
The data is simulated, so the true parameter of z both at time 1 and time 2
is 5, which is very close from the model output
for time 1, z = 5.0757760, and for time 2, z is 5.0757760-0.6379866 = ~5

model=gee(y~x+z+x*tind+z*tind, family=gaussian(link = "identity"), id=sid,

corstr="exchangeable")
Beginning Cgee S-function, @(#) geeformula.q 4.13 98/01/27
running glm to get initial regression estimate
(Intercept) ? ? ? ? ? ? ? ? x ? ? ? ? ? ? ? ? ?z ? ? ? ? ? ?tind
x:tind ? ? ? ? ?z:tind
?2.9342186 ? 1.5002601 ? 5.0757760 ? 2.0846327 ? 0.1869748 ?-0.6379866



However, when I use the summary command, the coefficients are changed.
Am I missing anything here ??