I'm not the grammarian you may be looking for. However, my thoughts are that the "Christ Is Ascended" terminology may be the Church's effort to demonstrate that everything we celebrate in our Life in Christ is presently occurring, rather than having happened in the past, and ending; ex. How many of our feast day hymns begin with "Today...?" While He did ascend, His ascention remains current. Admittedly, it may not be grammatically correct. Likewise, with "Christ Is Risen."
Again, I'm not sure, but this type of grammar may be an English language phenomenon. A Greek Orthodox priest once told me that "Christos Anesti," would be better translated as Christ has Risen. I don't know for sure.