I have had that happen to me also Ziza.. when you come back all your crashes are removed from the stats.. I don't know what the deal is with the kills.. perhaps the tank or chute kills are removed as well as the crashes?
This just happened to a red in a DF, hitzu, I came in and he was 8-0, I shot him down twice and he got sk'd too so he was at 10-5 then he left and came back but was 10-0!!!!!! Not cool!