When you add any number of the original cuts (presumably as constraints, rather than lazy constraints or user cuts) and then restart the solver, CPLEX will go through a presolve step, the results of which may be different from the original presolve due to the presence of the extra constraints.
It will then solve the root LP, the solution to which will likely be different from the solution to the original root LP, and so on.
So the branch-and-bound tree in the second run may well be significantly different from the tree in the first run. That can lead to CPLEX encountering different "feasible" solutions along the way, leading to different Benders cuts. Whether the second run reaches optimality with more, fewer or the same number of cuts as the number of unused cuts from the first run is largely a matter of luck.
Update: Oops! I thought the master problem was a MIP, when it was clearly stated to be an LP.
The master problem in the second approach will be different from that of the first even if presolve makes no changes in either of them. If CPLEX starts the second master at a different vertex from the vertex at which the 15th cut was found, it may lead to a different master solution, triggering a different cut, and so on. It might be interesting to record the master problem basis after the 15th cut is added and the master is solved again in the first approach, add the first 15 cuts, then start again using the recorded basis as the starting solution to the LP. I'm still not sure that would generate the same sequence of cuts (partly due to presolving, partly due to random tie-breaking whenever there is a tie for the best column to enter into the basis), but perhaps it would.