[BioC] problem with expresso() in affy package

[Chunrong Dong]

Dear there:

When I reduce probe-level data to expression-level data, I tried

     exprs_norm <- expresso (cels,
                 bgcorrect.method="none",
                 normalize.method="invariantset",
                 pmcorrect.method="subtractmm",
                 summary.method="liwong")

and 

     exprs_norm <- expresso (cels,
                 bgcorrect.method="none",
                 normalize.method="invariantset",
                 pmcorrect.method="mas",
                 summary.method="liwong")

Both gave me exactly the same result. It indicates that the two
pmcorrect.methods "subtractmm" and "mas" works the same which is not
reasonable. 

Could anyone help me with this please?

Thanks.

Chunrong