well the wastegate controls the boost

however the boost at the compressor wheel and at the inlet manifold can be quite different

consider these two senario's

1. a turbo supplying say 10 psi at the compressor may supply say 14psi at the inlet manifold if the engine can't consume the air - it will spike and drop back as the pressure at the compressor raises to suit and the actuator opens

2. and like wise a turbo supplying 10 psi may make 7 psi at the inlet manifold if the engine consumes more than it can supply (a bit like the mod to the crossover pipe on the rst)

I appreciate it looks like i'm being awkward here, but i'm genuinely interested to find out why surge can mean such different things to people in this industry