measuring jfet gate leakage
I don't see where you get the 10uA.
The range is given as 100uA to 10A. This does not mean the unit can only measure 100uA to 10A. It means, it has range settings that allows measurement from 0 to X.
Here's the table from extended spec:
Range Full Scale Resolution
(6½ Digits) 4½ Digits 5½ Digits 6½ Digits
100 μA 100.0000 μA 10 nA 1 nA 100 pA
1 mA 1.000000 mA 100 nA 10 nA 1 nA
10 mA 10.00000 mA 1 μA 100 nA 10 nA
100 mA 100.0000 mA 10 μA 1 μA 100 nA
1 A 1.000000 A 100 μA 10 μA 1 μA
3 A[1] 3.00000A 1 mA 100 μA 10 μA
10 A 10.00000 A 1 mA 100 μA 10 μA
As you can see, it has 7 range settings and at 100uA range setting, the FULL SCALE (MAX READING) is 100uA.
EDIT: Yes, I admit reading the extended spec, minimum accuracy in 100uA range setting is 20nA, but it still does not give what the minimum resolution is and it is still a lot better then 10uA you seems to suggest.