As found in the page for Fees
“IASC members (and other ISI section members)” has been replaced by “IASC members(other ISI section members, and participants of WSC2019)”.
That is, the registration fee for the participants who also attend at WSC2019 is that for IASC members.
The ones, who had already paid the fee other than that for IASC members in spite of attending at WSC2019, can describe this reason and email to in order to receive the repayment of the extra-payments.