There are two types of "pots". Linear and Log (logarithmic) - I assume log = a and linear =b.
Linear have a linear progression of resistance, and log it is logarithmic.
Thus the "all at one end" effect. Linear or log could create this effect depending on what the circuit is.
To explain the difference, if you divide the rotation of the variable resistor into 10 positions and measure the resistance at each of those positions the results would look a bit like (I hope this works):
Pos....Log...Lin
1.......1.....1
2.......2.....50
3.......4.....100
4.......8.....150
5.......16...200
6.......32...250
7.......64...300
8.......128..350
9.......256..400
10.....512...500
Ok the maths didn't work out but you get the idea...
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