I'm sorry this isn't logical (note: I don't say impossible). When you turn the engine through one turn without the cam in, nothing changes in relation to anything else, when it comes back to T.
Especially considering most optimised engines are not timed up with degree discs and dial guages. For example, I just have a graduated tool that measures the position of the cog on the crankshaft, I 'm sure other engine builders use variations of this.
Having said all of the above. So many things on these engines respond differently to how you would expect to be logical or possible, nothing would surprise me.
The only explanation I would have to what you experience, is the different distortion in the side cover when it is fitted and torqued up. This could easily 'pinch' the engine and take the edge of the revs.
Although peak revs have little bearing on max torque, bear in mind that they do give a good indication of peak power, so it is important to monitor this. For high rpm the curve tends to be linear, back from where the engine refuses to rev further. This is why valve springs are so rigourously controlled because they are one of the biggest controlling factors in peak revs and consequently peak power. You only really get a handle on this using a brake type Dyno. Even inertia dynos are not so good in this respect.
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