Thanks everyone (especially you slartabartfast for sharing your deep knowledge!).
After my previous post, I discovered that when I struck a bit softer (around 100-116), the black and white keys showed almost the same velocity values. So the problem area was between 116 and 127 (especially above 120). I reported this to Roland, and received their reply with a reason that I think convinced me.
Because they said earlier that they carefully check and adjust the black and white keys of any of their keyboard products before placing them onto the market, I asked whether they spent the same amount of time and energy on A-PRO as they did on their high-end synths like JUPITER-80 or VR-700.
And, it wasn’t the time and energy that made A-PRO's keyboard different. It was the material used for the keys’ chassis (frame). JUPITER-80 and VR-700 (and also other expensive ones, I suppose) use iron sheet for the chassis, while A-PRO (and probably other reasonably-priced ones) uses resin. Keys using resin for the chassis tend to 'bow' when hit hard, and this makes the 116-127 velocity range of black and white keys somewhat different.
(Sorry if I mistranslated their explanation.English isn't my native tongue. I used the word 'resin' for the first time in my life
)
So, I think I can now happily use my A-800PRO, because, fortunately, values in the 116-127 velocity range don't very frequently appear in most of my keyboard takes (e.g. organ, electric piano), so it won’t be a serious problem unless I record hard synth stub or lead synth (neither has many velocity layers so maybe not a problem). As for non-keyboard instruments like drums, bass, strings and brass, I don't mind about velocity very much since I can edit them freely without feeling guilty.
(BTW, someone from another forum recommended me JS plugins in Reaper (also included in free ReaPlugs, I think), which can allow users to program and make every black key to get +10(or whatever) on velocity in real time. Hmm..interesting.)