I've now got an answer. The term "Asukaspuisto" is an old name used for playgrounds in some districts of Helsinki. These are however no longer in use so both the name "Asukaspuisto Sanna" and the sign are outdated. They've once refered to this playground feature https://www.openstreetmap.org/node/299090081 which official name nowadays is Leikkipuisto Sanna.

I don't any longer remember whether it has been stated in this thread that the Urban Environment Division is responsible for naming parks and roads where as the Education Division is responsible for naming playgrounds. Obviously the synchronisation between the different name versions between these two Helsinki City divisions could have been better in this particular case.