Yes, your observation is true in a way. Some versions DO NOT work with each other. Very often however, different versions also DO work with each other. We try to keep them compatible whenever we can. Still it is only safe when always keeping newest most recent version on both sides.
The reason: spacedesk is still free and we still don’t have the budget to work on „automatic update“ feature and on comprehensive release management. Currently we still focus our development capacities on new core features. The next one soon to come: Audio playback support