The electoral system for the Senate was a strange hybrid which established a form of proportional representation into FPTP-like constituencies. A candidate needed a landslide victory of more than 65% of votes to obtain a direct mandate. All constituencies where this result was not reached entered into an at-large calculation based upon the D'Hondt method to distribute the seats between the parties, and candidates with the best percentages of suffrages inside their party list were elected.
No senator obtained a direct mandate. Please remember that the electoral system was, in the other cases, a form of proportional representation and not a FPTP race: so candidates winning with a simple plurality could have (and usually had) a candidate (usually a Christian democrat) with more votes in their constituency.