Colder is not necessarily better with Saccharomyces pastorianus (S. pastorniaus). Not all lager strains are equally cold tolerant.
There are two different families of strains in the S. pastorianus family, one of which is more cold tolerant than the other. Czech and Danish strains belong to the Saaz family. Saaz-type lager strains are triploids with two sets of what it is currently believed to be S. eubayanus chromosomes and one set of Saccharomyces cerevisiae chromosomes (S. cerevisiae). German strains tend to belong to the Frohberg family. Frohberg strains are tetraploids with two sets of S. eubayanus chromosomes and two sets of S. cerevisiae chromosomes. Most S. pastorianus strains have chromosome deletions and/or additions (aneuploidy). The genetic contribution by S. eubayanus is what gives S. pastorianus its cold tolerance. Saaz strains are more cold tolerant than Frohberg strains because their genetic make up is 2/3rds S. eubayanus.
With the above said, can anyone guess why W-34/70 is forgiving at higher temperatures than one would normally believe possible with a lager strain? If you guessed that it is a Frohberg strain, then you are starting to understand the impact that genetics has on yeast performance. In order to achieve best performance from W-34/70, fermentation temperature should not be allowed to drop below 50F (W-34/70 is half ale yeast from a genetic point of view).
By the way, our knowledge of S. pastorianus is rapidly changing. Diego Libkind's discovery spurred a new round of research into the origin of the yeast species. Two years ago, Saaz yeast strains were believed to be diploids with one set of what it is currently believed to be S. eubayanus chromosomes and one set of Saccharomyces cerevisiae chromosomes whereas Frohberg strains were believed to be triploids with one set of S. eubayanus chromosomes and two sets of Saccharomyces cerevisiae chromosomes. Improvements in genetic sequencing are leading to new knowledge of a yeast species that appears to have not existed before the 15th century.