I'm using SCID with the latest Stockfish 7 on my Ubuntu PC. When I'm analyzing a position, I can see the depth counter on the left. After seeing that the move, Nf3 say, get to depth 25, I go to play the move. I would expect that when the engine is analyzing this new position, it would start at depth 25, but this is not the case. The engine needs to work up to there from the first observed depth considering the exponential increase in time-computing depth, around 14 or so.
Am I misinterpreting the depth counts or is there some technical reason as to why this is happening? One reason I thought, might be because if I hadn't chosen Nf3, and chose one of the worst moves, then there would be many missing branches due to pruning between candidate moves. This would mean evaluating the move using previous information would save little time since few variations have been examined.