SEs rms cph vs survey svycoxph

I?ve used twang to get ATE propensity scores.

I?ve done multivariable, case weighted Cox PH models in survey using
svycoxph and in rms using cph with id(cluster) set to get robust estimates.

The model language is identical.

The point estimates are identical, but the CIs are considerably wider with
svycoxph estimates.

There is a note in the svycoxph help page stating the SEs should agree
closely unless the model fits poorly.

Are there other reasons for the differences?

Nathan