There are many ways to fit distributional parameters to data: method of moments, optimization of the likelihhod equation, optimization of an approximate likelihood, matching percentiles, and so forth. The estimates can also depend on the initial guess and the numerical optimization technique.
The documentation for the procedures descrbe how the parameters are estimated. The doc tor PROC SEVERITY shows how parameters are initialized and what estimation techniques are used. For example, for a gamma distribution, an APPROXIMATE MLE is formed by approximating the digamma function. This results in a fast estimation. In contrast, the UNIVARIATE procedure solves the true MLE by using the full digamma fnuction. It is slower, but more accurate.
In short, you are not going to be able to get exactly the same answers for the distributions for which SEVERITY and UNIVARIATE use different estimation methods. For distributions like the gamma, the documentation indicates that the estimates might be off by a few percentage points.
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