The procedure is performing two series of tests, that as @PGStats said are based on likelihood ratio statistics. The Type1 table shows the results for adding effects to the mean model, (using the full zero inflated model), and the Type1Zero table shows the results for adding effects to the zero inflated model (using the full mean model). The 2*LogLikelihood values are the same in the final row of each table, because in both cases it is based on the likelihood when all the effects in the mean model and all the effects in the zero inflated model are used.
To your original question, the 2*LogLikelihood values are the same in the tables for the effect “a” because it was the last effect you specified in both the MODEL and ZEROMODEL statements, so it corresponds to the last row of both tables.
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