It might be unwise to deduce too much from the name of the laboratory. It might be historically the fifth such laboratory to be established, or it might simply be an arbitrary name decided by a beaurocrat. The fifth name on a list. Just call it "Signal Processing Lab 5"
These are the signal processing laboratories of EPFL, The École Polytechnique Fédérale de Lausanne:
Signal Processing Lab 1 - LTS1
Signal Processing Lab 2 - LTS2
Signal Processing Lab 4 - LTS4
Signal Processing Lab 5 - LTS5
Multimedia Signal Processing Group
http://splabs.epfl.ch/