You're only considering "always round" perfectly rigid wheel and tyre combinations, with no deformation and no change in grip provided. You seem to be following a simple F=uN type model, which we know does not in any meaningful way apply to tyres because, even by instinct alone, we know that wider tyres do give greater grip for instance.
Lets for simplicity consider a tyre that is rigid in the vertical plane but flexible around it's circumference:
At 100/100 the contact patch, with no acceleration/decceleration, is as normal (and perhaps as you view it).
At 100/0 (locked) ahead of the contact patch is slightly stretched rearward all the way to the contact patch's trailing edge, overhanging a little.
At anything in between, the tyre bunches at the leading edge of the contact patch and overhangs rearward of the contact patch. All to varying degrees depending on the characteristic of the tyre. At a certain level of deformation, the tyre provides it's greatest grip.
Of course, throughout all of the above the shape and size of the contact patch changes considerably.
This is, very roughly speaking, how the tyre "knows" how much it is "slipping". Slipping gives the wrong impression of what is going on because it implies that the tyre is sliding over the surface in some way, which isn't necessarily the case.
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