JSampler Fantasia's keyboard always shows the range of an instrument an octave lower.
For example, given the grand piano of the Sonatina Symphonic Orchestra, the range displayed on the keyboard is C0 to C7. However, the samples range from C1 to C8 (and there are no mistakes in the corresponding SFZ file):
Keys - Grand Piano
Fantasia's range: C0-C7
samples' range: C1-C8
I am using version 0.9.
Please note that there are several different octave numbering schemes out there regarding MIDI notes. Especially there are two rivaling ones, which differ by exactly one octave number (IIRC one starting with 0, and the other starting with -1). To my knowledge there is currently no such thing as "the right one" for numbering octaves.
I am closing this report now.