The crankshaft has a gear on the end of it called the "Primary Drive gear" this gear directly connects to its mate called the "Primary Driven Gear".
The primary driven gear is mounted to the outter clutch housing, sometimes called the "clutch basket". The clutch basket drives the the inner clutch hub, this is sometimes called the "Clutch Boss"
The rotating energy of the clutch basket is transfered to the boss via the clutch plates.
The boss is center splined to the transmission's primary input shaft, the gears on the input shaft is center splined to that shaft. The teeth of those gears engage with the teeth of the gears that are center splined to the transmissions secondary output shaft sometimes called the "counter shaft". On chain drive motorcycles the front sprocket is called the "counter shaft sprocket" and it mounts to the outside very end of that shaft, then the sprocket chain goes to the rear sprocket.
I said all of that to say this,
If the crankshaft is turning at whatever RPM, then the primary drive gears are doing what they do and have no way to change their rate of rotation.
The transmission gears turn at whatever rotation, and the counter shaft sprocket turns at its rotation,
the chain does what it does,
the rear wheel turns.
Due to all the gear engagements, the only place were an RPM difference can happen between the RPM's of the crank shaft and the rear wheel and in the same selected transmission gear is at the connection of the clutch basket and the clutch boss (clutch plates)
ten horse power,
ten thousand horse power,
thick oil or thin oil.
The gearing can't change with out gears being broken. Its a law of physics that is written in stone!
A vehicle may run smoother, my feel more powerfull, but the engine RPM's at a certain speed in the same gear with a manual transmission can't change because its all set up by "gear set ratios" which means that each gear is engaged to its mate, but the two gears are a different size so each has a different amount of teeth and turn at a different rotation rate. A gear set CAN have a 1 to 1 ratio (same size, same amount of teeth), that happens when energy from one shaft is needed to be transfered to another, and without a higher or lower gearing of the gear set, but its still a FIXED engaugement!
Now, if your clutch wasn't slipping and you had an indecated difference in RPM's in sixth gear at the same speed, then your tach is not working right.