Well, they only instituted this particular award in 2010, which probably impacts the choices. The four winners have been LLVM (2010), GHC (2011), Jikes (2012), Coq (2013), and GCC (2014): http://www.sigplan.org/Awards/Software/Main
It's hard to tell from that list if the SIGPLAN award will end up being more retrospectively oriented, but a lot of ACM awards are. For example the Software Systems award does sometimes go to contemporary systems (LLVM won in 2012), but also some that are hard to justify except based on retrospective impact (Make won in 2003, Eiffel in 2006): http://awards.acm.org/software_system/year.cfm
I think in practice the decision process often ends up being: did anything come up in the past year or two that's really great? If so, give them the award. If not, look back for something older and influential that hasn't won yet.
It's hard to tell from that list if the SIGPLAN award will end up being more retrospectively oriented, but a lot of ACM awards are. For example the Software Systems award does sometimes go to contemporary systems (LLVM won in 2012), but also some that are hard to justify except based on retrospective impact (Make won in 2003, Eiffel in 2006): http://awards.acm.org/software_system/year.cfm
I think in practice the decision process often ends up being: did anything come up in the past year or two that's really great? If so, give them the award. If not, look back for something older and influential that hasn't won yet.