The range shown in the last link is correct for almost any standard piano.
There are models with a narrower range (like those small pianos seen in living rooms of many american films and tv series, usually spanning 5-6 octaves), as there are others with a bigger extension (notably, the Bösendorfer Imperial which has a full lower octave extending to the C below the standard A0).
That said, most pianos have an 88-notes range, starting from A0 up to C8.
The reason for the samples you provided might vary.
Consider that sampling techniques can be very different: depending on the requirements and technical skills of the "sampler" (those who actually record the instrument, available microphones, etc).
"Low quality" piano samples often have just a few notes for each octave (sometimes every three half tones, but I've seen some with just one note per octave), while better samples have each note recorded and for many different levels of "velocity" ("dynamics").
One of the most important limits of sampling is that the more accurate sampling is (more notes, more dynamics), the more size it requires, which can be very important for performance, since a sampler engine usually loads all samples for every instrument; this means that loading time can be very long, and it occupies a lot of system resources.
Since lower notes are rarely played, those who created that sampling might have decided to decrease the memory footprint of the library by leaving out "less used notes". Maybe it was done just for convenience, or after careful analysis of the result (by comparing the given samples at different dynamics).
Consider that lower notes also have a much longer sustain (meaning: they occupy a lot of disk space). Take for instance the pianissimo of the Bb0 compared to the C8: the latter only takes 1.1mb, while the former is 6mb. Adding all levels for all (very rarely used) notes would have increased the size by at least 20-25mb.