It is probably just the +/-15g on the rubber. It may just be that it's one of the extremes. Try testing out how much force is needed by stacking coins. The feeling of one of the keys being softer than the rest is not unheard of. Nearly all of the keys on the right side of my HHKB feel soft and lack a bump, but I'm trying to get a replacement. If it really bothers you, because it's so light, try contacting the seller to see if you can get a different one sent to you, because some people don't mind keys being lighter than others, but some can't stand it.
Good luck!
P.S I may be wrong. It may be something else causing the feeling, but the 15g tolerance is likely to be the problem.